Mountain bikers often travel along dirt roads, wide enough to pass one another. But some off-road trails are little more than a narrow rut, formed by dozens or hundreds of bikers over the years; that's singletrack.
On singletrack, you'll generally have no choice about your route: You've got to stay in the track. There may be rocks, roots, tight turns, and puddles to negotiate. If the track is deep, the pedal will hit the dirt if you time your downstroke wrong. In short, singletrack adds new challenges and excitement to mountain biking. But it's not the best terrain on which to learn.