Arbitrage, or arbing, exploits differences between the prices offered by separate bookmakers. If the implied probabilities of all outcomes add up to less than 100%, bets can be spread so the same amount comes back whichever outcome happens. In practice the gaps are small and close fast. Prices can change before every bet is placed, bets can be voided or limited, and mistakes in stake sizing can turn a planned profit into a loss. Bookmakers may also restrict accounts that bet this way.
Example
In a two outcome market, two bookmakers offer decimal odds of 2.10 on opposite sides. The implied probabilities are 47.62% each, 95.24% together. Staking 50 on each returns 105 from 100 staked, a margin of 5%.