We find the tritone between notes B and F.
In the Db7 chord:
The tritone is found on notes F - Cb. As you may have noticed, these chords share the same tritone (F = F and B = Cb):
Because of the fact that they share the same tritone, one chord can substitute the other. This is called in jazz music theory tritone substitution. Note that the chords are one tritone apart (G - Db).
A short example where the G7 chord is substituted (shown in red):
Now the second chord in each measure is substituted using tritone substitution:
A very interesting use of this progression can be found in Chopin's Prelude #8.