The next section (#11-20) of the Modern Jazz Line series will cover the tri-tone substitution. A "tri-sub" within a ii-V-I involves substituting a Dominant 7th chord a tri-tone away from the V7 chord. The concept is explained simply in the video below.
Simply put, you can substitute a Dominant 7th chord a tri-tone away from the V chord (in a ii-V-I for example) because they share the same guide tones.
G7 = 3rd and 7th = B and F
Db7 (tri-sub) = 7th and 3rd = B and F!
If voiced correctly the two chords can and will function essential the same in a ii-V-I sequence. If finding a tri-tone away from the V7 chord is hard at first just think a half-step below the ii chord. I'll show you what I mean.
ii-V-I in C Major = Dm7-G7-CMaj7
with a tri-sub on the V7
ii-V-I in C Major = Dm7-Db7-CMaj7
Notice the root movement of the new progression. Each note simply moves down in half-steps. Feel free to use this method if it is easier for you to remember.