Without giving examples, I think a Drum & Bass roller is: A track which key elements are the drums and the bass, predominantly consisting of a constant and repeating pattern with linked basslines or drums which seem to go on forever.

What does roller mean in DnB?

Essentially, a roller is any track with a rolling bassline or beat structure. It's not to do with a time signature, as some have suggested; it can be literally any time signature as long as it's a consistent loop of bass, drums or both (sometimes even synths) which roll over and over throughout the track.

What does DnB set mean?

Drum and bass (also written as Drum&Bass or drum'n'bass and commonly abbreviated as D&B, DnB, or D'n'B) is a genre of electronic music characterized by fast breakbeats (typically 165-185 beats per minute) with heavy bass and sub-bass lines, samples, and synthesizers.