A riff is a repeated chord progression or melodic figure, often played by the bass instruments, that forms the basis or an important part of a rock music or jazz composition; an ostinato figure.
Examples of music that depend on riffs for their effect:
... the sufferers of Hansen's disease (leprosy) who lived on the island of Molokai, Hawaii.
He was born in Tremeloo, Belgium, the son of a farmer. He entered the novitiate ...