What it is
Maracas are a pair of rattles on handles: hollow shells (once gourds) with seeds or shot inside.
They're among the oldest instruments of son. In salsa they're often played by one of the singers in the chorus.
The strokes
There's one sound: a short "sh" from a sharp flick of the wrist, as the seeds hit the shell.
The hands alternate, so the maracas sound evenly on every eighth note.
What a dancer hears
Even eighths are the grid everything else sits on. When the count is hard to hear, the maracas help you feel the & between counts and keep your steps even.