A bookmaker, or bookie, offers prices on the outcomes of sporting and other events. Instead of trying to predict winners, a bookmaker builds a margin into the odds so that the implied probabilities add up to more than 100%. Over many bets, that margin is the bookmaker’s expected profit. Bookmakers adjust prices as bets arrive and as news changes. Legal bookmakers need a license where gambling is regulated. This site explains how bookmakers work and does not link to or promote any.
Example
On a two outcome event, a bookmaker offers 1.91 on each side. The implied probabilities are 52.4% each, 104.7% in total, so the margin is about 4.7%.