That’s as a “Relative Minor” chord. This is a minor chord that is built on the same scale as a major chord, so the two are “related.” For example, G major and E minor scales use exactly the same notes. If you know how to play a G scale, then you can start on the E note and play exactly the same notes as the G scale but end up playing the E minor scale.
If you don’t know the scale, you can still play around with the chords. Because they use the same scale, the relative minors of chords you already know are relatively easily to play. Here are the relative minor chords.
|C -> Am||C#/Db -> A#m/Bbm|
|D -> Bm||D#/Eb -> Cm|
|E -> C#m/Dbm|
|F -> Dm||F#/Gb -> D#m/Ebm|
|G -> Em||G#/Ab -> Fm|
|A -> F#m/Gbm||A#/Bb -> Gm|
|B -> G#m/Abm|
In most songs where you hear a minor chord, it is usually one of the relative minor chords of the main three chords. Most often, it is the relative minor of the I chord or the IV chord. If you want a different sound for the V chord, you usually use a seventh chord.
Just for fun, you can go back and play all the songs you know but play the relative minor instead, or just add it for half of the beats.
For example, instead of doing:
C / F / G / C / or G / / / C / G / do: C Am F Dm G G7 C / or G / Em / C Am G Em