Prior to the "one drop" beat, the bass drum would typically fall on the 1st and 3rd beat and the snare would fall on the 2nd and 4th leaving the high-hat to accent throughout. However, on a "one drop" beat, the bass drum is used only on the 3rd beat, leaving an open space on beat one, hence the name "one drop". Said to be invented by the original Wailers' drummer Carlton Barret.
Made popular on Bob Marley & The Wailers' song by the same name.
And well still find time to rap;
Were makin the one stop,
The generation gap;
Now feel this drumbeat
As it beats within,
Playin a riddim,
Resisting against the system"
"One Drop" by Bob Marley & The Wailers