But it mainly means improvisation, making new things up, using your imagination.
Jamming can be just improvising by yourself on a song. I generally use it to mean playing with other people. But “playing” in the fun sense of the word, not rehearsing and rehashing the same song but coming up with something new. Of course, you need to know the melody before you can improvise around it and everyone has to play the same chords (or similar ones) over the same rhythms to form a bed of sound over which the soloist or two or three can improvise.
Jamming can also mean cluttering up radio signals so you can’t make sense of their sound and that connotation often arises in jam sessions, too. That’s okay. You can’t make music without occasionally just making noise.
That’s what a jam session is supposed to be: Having fun.
At a regular jam (as opposed to a beginner jam), the rest of the musicians are keeping the time and playing the tune. All you've got to do is sit in the back and try to keep time quietly. Beginners aren't expected to contribute. This is a lot different than sitting in front of a teacher (an expert in your mind) and playing by yourself or even playing along with an expert.
I don't think going to a beginner jam so you know they'll put you on the spot is necessarily a good thing. Go to a regular jam, sit in the back, and play quietly. It's far more relaxing than any lesson. You'll still feel uncomfortable, but it won't be like a sinner going to church to get judged by experts; it'll be more like the discomfort at the first day of work at a new job where everyone knows you're new and everyone will offer some advice and some of the folks will even offer worthwhile advice, but you won't know what's what or who's who for quite awhile but at least there aren't a lot of expectations and you'll get a paycheck at the end of the week.
You ought to know at least 10 chords and have a capo if you’ve got a banjo or guitar. On a mandolin, you’ll need to know the basic chord forms (4-finger chords) and move them around on the neck. The dobro and autoharp are special cases.
It helps to be able to hear how you sound, but that’s a skill that is often learned quicker at jams than while playing by yourself.
Just as there are different types of conversations, there are all varieties of jams. Some are open to newcomers just like a bar argument about the local sports teams or the weather. Some are only for experts such as talks about muons in a college corridor or a barroom discussion of the relative merits of a hemi versus a straight head piston. You can listen but don’t join in unless you know what they’re really talking about.
Look for advertisements for jams on the bulletin board at music stores. Those are almost always open to newcomers.
Show up with your instrument in its case and listen to the music. Someone may come over and invite you in.
If they don’t but it sounds like music you could play, then
Some people think you should always ask, but in the hundreds of jam sessions I’ve been to, only a few have had actual leaders (and I’ve never liked those anyway—does a conversation have a leader?). Most jams are collections of folks who know each other and newcomers. Since you’re new, you don’t know who the right person to ask is.
If the music seems like you could play it, then treat it like a conversation and speak up.
But don’t be obnoxious—loud, out of time, out of tune, off-key. Be polite and listen to the others and see if you’re making things better or at least not making things worse.
If they ask you to quiet down, do it They’re open to training newcomers in the art of jamming.
If someone gets mad at you and tells you to quit, then leave. You don’t want to hang out with that type of person anyway. But come back to the next jam. That person may not be there or he may be in a better mood. Don’t write off a jam until you’ve gone to it at least three different times. The personnel usually changes.
If you keep coming back often enough, you'll become a regular. People will talk to you and may ask to you to come up front. This will also occur because you're getting better and getting used to the tunes that the jammers play. This can also happen if you stay late enough and the jam thins out. Making a judgment on a jam based on one visit is not a good idea because they change depending on the players. Another thing that may happen if you show up often enough and sit quietly playing along is that some other beginners may sit next to you and you'll make some new friends and maybe start your own jam.
Lead a song even once and you'll realize that the leaders of a jam song have a lot going on to make things work smooth. Help them out by not taking offense. It's not easy to hit the grand finale of your break while also looking around to see if anyone else is ready to take a break, especially when they're all looking at their own fretboards. That's one reason why it's easier to go around the circle, but sometimes you want to break things up instead of having four banjo breaks in a row.
Some jams frown on having music. Many don’t care. Whatever works to make music is good. One problem with bringing music is that it becomes a crutch and you’re not really playing the song. The advantage is that you don’t need to memorize the chords. But memorization leaves you free to listen to what’s going on.
I’ve found that beginners shouldn’t bring music. Memorize the songs you want to lead. Know them cold. If someone else has music, then you’re going to spend a lot of effort reading when you could be playing and listening. Experienced pickers can play from sheet music and still listen to what’s going on because of all their experience.
A beginner needs to learn to get rid of the crutches. Bring the minimum needed to get you through the song. A chord chart and first word to every verse ought to be the starting point. Nothing at all can be the end of the process.
Relax. Smile a lot. Put the other folks at ease by not being uptight yourself. It’s just music.