The tritone is an interval of 3 whole steps and naturally present in the dominant 7th chord between the 3rd and 7th of the chord. If we look at the G7 dominant chord (G, B, D, F), we see notes B and F form a tritone.

Each tritone can be inverted, so B and F in reverse becomes F and B. The dominant chord that shares the same tritone with the dominant 7th chord is 3 whole steps away and is the tritone substitution. For example, 3 whole steps up or down from G leads you to Db. The tritone in reverse as F and B occurs in the Db dominant 7th chord.

The tritone substitution is very often used in place of the dominant chord in the ii-V-I progression. It is when you substitute the V (G7) with its tritone (Db7) whose root is 3 whole steps away, because both dominant chords share the same tritone B and F. The tritone substitution is a half step above the resolution one chord (Db is a half step above C). So, focus on your resolution chord and lead into it with a dominant seventh chord one half step above.

https://youtu.be/Kh5AHETsrV0