Principle
In G7, the tritone is formed between B and F. The Db7 chord contains the same notes (F and Cb=B). So Db7 can replace G7 when resolving to C.
Chromatic bass motion
The main advantage is chromatic bass motion. Instead of G → C (fifth down) we get Db → C (half step down). This creates smooth, elegant voice leading.
Tritone sub in II-V-I
In the jazz cadence Dm7 – G7 – Cmaj7, replace G7 with Db7: Dm7 – Db7 – Cmaj7. The bass moves chromatically: D – Db – C.