Q: and how do I substitute it?
A: "tri-tone substitution" would be like let's say if you have a normal song in the key of C, and you have that D7 chord, which famously contains the notes F# and C in the chord. If you lie around thinking about music for long enough you'll notice that you can play the F# and C notes in the chord, with the D bass note, and that gives you the D7 chord, but you can also put the bass on Ab instead and that gives you an Ab7 chord just the same. So a "tri-tone substitution" is when you use Ab7 instead of D7. (or A7 instead of Eb7. or Bb7 instead of E7. and so on). seems crazy but it just might work