What it is
The tres is a Cuban guitar with three pairs of strings, hence the name. It's the lead instrument of son, the music salsa grew out of. A salsa band doesn't always have one, but its metallic ring is easy to recognize.
The strokes
The tres plays single chord notes and dyads — two notes together.
The guajeo is a pattern that fills the gaps the piano leaves. It has the same push on the & of 4, and no stroke on the 1 after it.
What a dancer hears
When the piano and the tres play together, their parts mesh like gears. For a dancer the tres is the sound of old-school, son-flavored salsa; in son it leads the whole band.