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."
an mc might say, "Yo me and
my boy were in a cipher last
night and we just started to riff off each other, we were just lettin it flow back and forth, ya
know what I'm sayin."