Consider a tennis match where the following occurs:

Player 1: The best price available is 6/5
Player 2: The best price available is 5/4

If you bet 50.56 on Player 1, you will receive 111.24 (including your stake) back if he wins.

If you bet 49.44 on Player 2, you will receive 111.24 (including your stake) back if he wins

Therefore you have invested a total of 100 (50.56 + 49.44) but you will receive a total of 111.24 regardless of which player wins.

The 11.24 is the risk-free arbitrage profit earned on the trade.