If one book has Team A at +110 and another has Team B at +110, you can bet both sides and lock in a guaranteed profit (the implied probabilities sum to under 100%). Arbs are rare, small (usually 1-3%), and the primary reason books limit accounts — arb hunters are easy to spot.
Advanced concepts
Arbitrage
Betting both sides of a market across different books at prices that guarantee profit.
Example
+110/+110 two-sided: bet 50% on each side. No matter which wins, you profit ~4.8%.