What it is
Congas (tumbadoras) are tall Cuban drums with a skin head, played with the hands. A conguero usually has two or three drums of different sizes in front of them, sitting or standing between them.
In salsa the congas play the tumbao: a short figure, one bar long, repeated through the whole song. It is the band's pulse: other instruments change parts, the tumbao stays.
The strokes
Heel and toe: the hand rocks on the head, first the base of the palm, then the fingers. They're quiet, nearly lost in the band, but they keep the hand moving and the eighth notes even.
Slap: a sharp, cracking stroke with the fingers near the rim, on 2 (and 6).
Open: the fingers strike near the rim and bounce straight off, so the head rings out as a note. The two open tones on 4 and & (8 and &) are the tumbao's "boom-boom".
What a dancer hears
The two open tones tell you where you are in the bar: right after them comes 1 or 5. Lost the count? Find that "boom-boom" in the music and come in on the next beat.
The slap on 2 and 6 suits those who dance On2: their break, the change of direction, lands on it.