The following are a few examples of chord substitution.
The 1 (I) can be substituted with the 3 (III) or the 6 (VI)
The 4 (IV) can substitute for the 2 (II)
The 5 (V) can be substituted with the 7 (VII)
Flat-five substitution = substituting the dominant 7th on the flattened 5th of the original chord