A triplet is three notes in the space of two, that's all there is to it. I don't recall how that song goes, so I can't say how well that applies to Cliff's playing though.
For example, if you normally had one bar of written rock music where the bass keeps a steady stream of, say, eigth notes, then you'd be playing 8 notes per bar (in 4/4 time). If you were to play eighth note triplets instead of eigth notes, you'd be playing 12 notes per bar. You don't have play entire bars as triplets, of course, but that should apply to Ride the Lightning. Reply and say if any of that made sense
