In music a riff is a repeated
instrumental melody in a song like that catchy guitar lick from Black Sabbath's "Iron Man." It also means to improvise in your
performance, esp. by expanding on something recognizable. So another artist might start with Sabbath's "Iron Man" melody at a live show and then take it in completely different direction.
"Man that's a sick guitar riff in that Metallica song Enter Sandman"
"I saw this great improv jazz band that could really riff."
The term is also often used to describe inspiration and
improvisation in other things like acting, comedy, dancing, drawing, djing/scratching etc.
a b-boy might say, "So, I was breakin with Jamil the other day and was feelin what he was doin so I started riffin off his style and adding some new flava to it. I vibed off his uprock but threw in some popping and dropped down to a wind mill...It was dope."