What it is
The güira is a metal cylinder punched with bumps, scraped with a metal brush. It's the Dominican cousin of the güiro. In bachata and merengue it keeps time, like a hi-hat in pop music.
The strokes
Short: a quick, short scrape. Long: a scrape along the whole length.
In the derecho (verse) the güira plays even short scrapes on every eighth; in the majao (chorus), long ones on every count; in the mambo, a strict pattern: 1, the & of 2, 3, the & of 4.
What a dancer hears
The güira is bachata's metronome. In the verse it ticks off eighths; in the chorus it moves to long scrapes on every count, and the music seems to open up. A change in the güira tells you the song has moved to another part.