What it is
The bass in salsa is an upright bass, a bass guitar or a "baby bass" (an electric upright). It carries the harmony with the piano and holds the rhythm with the congas.
The strokes
The bass plays the root and fifth of the chord.
The bass tumbao: the fifth on the & of 2, then on 4 the next chord's root, held over the bar line. That's why the salsa bass almost never plays the 1.
In cha-cha-chá, pachanga and bolero the bass is simpler: the root on 1, the fifth on 3.
What a dancer hears
A bass that skips the 1 throws beginners off: the lowest note lands on 4 and rings into the 1. Don't look for the 1 in the bass — it's right after the long note on 4 (and 8).
In cha-cha-chá it's the other way round: the steady bass on 1 and 3 is a great landmark.