A tritone substitution swaps a dominant seventh chord for another dominant seventh whose root lies a tritone (six semitones) away. In C major the V chord G7 (G–B–D–F) can be replaced by D♭7 (D♭–F–A♭–C♭). The trick works because the two chords share the same unstable tritone: G7 contains B and F, and D♭7 contains F and C♭, which is the same key on the piano as B. Since that interval is what drives the chord towards its resolution, the substitute leads to C just as well.
The audible result is a bass line that slides down by semitones. Instead of Dm7–G7–Cmaj7, with the bass leaping D–G–C, a player can use Dm7–D♭7–Cmaj7, with the bass stepping D–D♭–C. The sound is slightly more sophisticated and less predictable, typical of bebop and later jazz, and it often turns up in the turnarounds of jazz-flavoured lo-fi loops.