Bitcoin Arbitrage is taking advantage of the price different between 2 exchanges. Say if exchange 1 has buy orders for 100$. And Exchange 2 has sell orders for 99$. Then you would buy from E2 and sell at E1. For every bitcoin you will get that 1$ extra, which is arbitrage.

For discussions regarding arbitrage found this thread : https://bitcointalk.org/index.php?topic=528691.0 .