It's just a black-painted metal tube that sticks onto the light where the barndoors normally go to allow more of a narrow spotlight to hit the subject. Usually there are rings of different hole sizes to control the size of the spotlight. It won't be the same sort of perfect circular spot that a stage Leko ellipsoidal lamp creates though, but on a fresnel flooded out, you can get a fairly decent circle pattern with a snoot if you want.
One use of a snoot might be to put a tiny spot of light on the actor's eyes, maybe even with some black tape on the end of the snoot to cut it further.
