One drop is the most common drum beat used in reggae. Almost all of Bob Marley's songs are one drop. The name comes from the bass drum pattern which does not land on the one, or downbeat of each measure. This is a big part of the reggae sound.
There is some debate over how one drop should be counted. The problem is where the guitar skank falls. In ska music the skank fell on the upbeat of every measure. When Jamaican music slowed down through the rocksteady and reggae periods, the feel of the skank moved more to the two and four of every measure.
Currently I feel the one drop should be counted as:
1 (drop)
2 skank
3 bass drum
4 skank
But it is also possible to count it as:
1 (drop)
& skank
2 bass drum
& skank
3 (drop)
& skank
4 bass drum
Either way could be considered correct.