In jazz, II-V-I refers to a minor 7th chord built on the second degree of the scale leading to a dominant 7th chord built on the fifth degree resolving to the Tonic built on the first degree, the "Do", the root, etc.

These II-V-I patterns are shown as arpeggios, but they can be strummed or plucked as a chord as well. These are great for soloing and comping in a sparse verse or chorus to vary the texture of the harmony.