What it is
The güiro is a hollow gourd with notches cut in it (today often wood or plastic). A stick scraped along the notches gives a rasping sound.
It came to salsa from Cuban son and danzón, and is heard best in cha-cha-chá and the montuno.
The strokes
Long: the stick runs along all the notches, and the sound draws out. Short: a quick, short scrape.
The pattern takes two counts: a long scrape on 1, two short ones on 2 and its &. Then the same on 3 and 4.
What a dancer hears
The long scrape draws out from an odd count to the even one; the short "ch-ch" falls on 2 and &, 4 and &.
In cha-cha-chá the short ones on 4 and & land right on the dancer's "cha-cha".