Chord substition is a great way to get some extra millage out of the chords you already know. It’s also a great technique to find new ways to come up with interesting progressions. The basic idea is to use one chord in place of another.
Let’s start with a few simple examples.
C6 = Am7
Cm6 = Am7(b5)
With these two examples, they are the same because they share the exact same notes.