The Tritone Substitution is also referred to as the “Flat Five Substitution”.
When Reharmonizing, ( Substituting chords ) a Dominant chord can be replaced with another whose root is a Tritone away.
By using this substitution, you change the ii - V7 - I cadence; to ii - bII7 - I.
Let’s take for example the key of C.
…Dm - G7 - C… becomes: …Dm - Db7 - C

You’ll find that the more accurate symbols for the bII7 are:
Sub V7/I.
SV7/I. (indicating that the V7 of I, is the chord being replaced.