What it is
Bongos are two small drums joined together: the higher macho and the lower hembra. In bachata they're the main percussion: the bongocero drives the rhythm and decorates it.
The strokes
High: a stroke on the macho. Low: an open stroke on the hembra. Slap: a sharp accent.
In the derecho (verse) the bongos play the martillo: every eighth on the macho, the hembra on 4. In the majao (chorus) they drop the upbeats: macho on 1, 2, 3 and hembra on 4.
What a dancer hears
The low hembra stroke on 4 and 8 is the main landmark in bachata: it's where the tap of the basic step falls. Lost? Wait for that stroke and start on 1 right after it.