Check out his link, but just so you know, that has nothing to do with chord substitution. Chord substitution is substituting one chord for another with a different root. I.E. If the song says play Ab7, I could substitute a D7 for a dom7,9,b5 sound. It can also be adding a chord change where there is none written. I.E. if the song calls for one measure of Bbmaj7, then one measure of C min7, I could play a half measure of Bbmaj7, a half measure of Bdiminished, and then a measure of C min, because the B diminished creates tension and wants to resolve to a chord whose root is at C. I know this probably isn't helpful, but I just want you to know that that's what chord substitution is.