What it is
The bass guitar in bachata sets the harmony and the bottom of the rhythm, together with the segunda.
The strokes
The bass plays the root and fifth of the chord: the root on 1 and 3, the fifth on the & of 2 and on 4. The first note rings for a count and a half.
What a dancer hears
Unlike in salsa, the bachata bass plays the 1 — you hear it straight away. The note on 4, with the bongos' hembra, underlines the tap.