Chord substitution involves playing a different set of chords for a given chord progression. This is useful for both beginners and advanced players. Beginners will be able to substitute easier chords for a given chord line (e.g. working from sheet music). Advanced players will be able to embellish a chord progression, often creating a 'jazz' flavour.
This table summarises common substitutions for beginners. The substitutions work because the chords contain similar notes and hence have a similar sound.
A chord progression simplified according to the chord substitution table.
More advanced players can use a reverse of the above table. For example, for Am play Am7, Am6 or perhaps use a combination of the two. This type of substitution is called direct substitution.