Scouts are members of the military. One of their duties is to keep cleansed and reforested lands from becoming reinfested with beasts, trolls, and giants. On the other hand, Sigrun, who is a captain of a troll hunting squad, seems to have experience in working with scouts as well.
The only known scout - more precisely, a night scout - we've seen in detail so far is Lalli, who doubles as a mage. Hence, we cannot be sure how typical his tasks and properties are of the scout profession, or of his personal character. A does, however, suggest that all young scouts there share Lallis dislike of communication with other humans, be it verbal or in writing.
During the preparation of the mission, Trond states that a cat - which they had originally on the roster - would "basically be just another scout". It is not quite clear how seriously this statement should be taken. However, part of the reason cats make good companions is because they can sense "foul creatures" before normal humans can, and on , Sigrun suggests that a heightened sensitivity to the presence of "anything sinister" is a common trait of scouts.