The ii–V–I (“two–five–one”) is a three-chord progression named with Roman numerals for the scale degrees its chords are built on; lower case marks a minor chord. In C major it is Dm7–G7–Cmaj7. Each root falls by a fifth (D to G to C), and the third and seventh of each chord move by a single step to the next, which makes the progression sound smooth and inevitable. In a minor key it changes colour: Dm7♭5–G7 (often altered)–Cm.
Hundreds of jazz standards, from “Autumn Leaves” to “Tune-Up”, are built from chains of ii–V–Is moving through different keys, so improvisers practise them as a basic vocabulary. In jazz lo-fi the progression often appears in a richer form, such as Dm9–G13–Cmaj9, looped over a relaxed beat: the cycle resolves just enough to feel settled, then starts again.