What it is
This is the most common chord progression in Jazz music. Knowing how to solo over it will be the most useful skill you can have as a jazz guitarist. First, let's look at the progression itself.
In the key of C, a ii-V would be composed of the following chords: Dmi7 and G7.
Typically, this is followed by the I, which in this case is Cmaj7. (Notice that in jazz, the 7 is automatically added to virtually every chord.) Both the ii chord and the V chord are organic to the original key; that is, you don't have to add any accidentals to create the chords. Let's change keys and see what a ii-V would be in Eb:
Hopefully, that will show you what a ii-V progression is. Anytime you see a minor 7 chord followed by a dominant7 chord whose root is a fourth higher, it is a ii-V. The chords themselves can also have added tones, like Dmi9-G13.
D Dorian mode, used over Dmin7
G Mixolydian mode, used over G7