A sleb is a celebrity — a famous person, the kind magazines put on the cover and gossip about. It is British informal, a lazy, slightly jokey way of saying celeb. Tabloids and glossy magazines use it; you would not put it in a school essay or a formal letter. It can sound a little teasing, as if fame is a bit silly, not a title of respect. Though actually, many British speakers still just say celeb; sleb is the more slurred, magazine-y twin, and some people find it dated now.