A layman might have a small amount of knowledge about something, but she isn't officially trained in the subject. If you need brain surgery, don't consult a layman; stick to a professional brain surgeon.
Use the noun layman to describe your brother, who is an amateur bird watcher rather than a professional ornithologist. The word layman is most often used to contrast with expert or professional. Another way to use it is to distinguish between a member of the clergy — a priest or minister, for example — and an ordinary church member, or layman.