You should put the word in context because It could many different things. I think the main meaning is a short melodic line played over and over that acts as a harmonic base for other playing or solos. (it's a musical term)

There's a few definitions for "riff", but aside from a musical riff, it usually means an improvised outpour of creativity. "To riff" would mean performing this outpour. But.... for your sentence, it would probably mean a short, snappy comment. Judging from the actual comment, the sentence is probably ironic. ;) Makes me think of a tiny dog: "riff! riff!!"