A riff is a pattern of notes (usually one, two or four bars long) which repeats throughout a verse or section of a song. A riff can also be varied to fit a chord progression. The use of riffs is very common in Blues playing. Here are some examples.
The following 12 Bar Blues contains a riff which is altered to fit each new chord as it occurs. As with previous examples, play it slowly at first and gradually increase the tempo.