Arbitrage profit is profit derived from a riskless (or near riskless) transaction.
For example, say gold is selling on the London exchange for $800 per oz and gold is selling on the New York exchange for $804 per oz. Buying one oz of London gold and selling one oz of New York gold (trades in close proximity) provides an arbitrage profit of $4 (less transaction fees).
The purchase and sale will likely have the effect of increasing the price of London gold and decreasing the price of New York gold. So for every subsequent trade, the arbitrage profit will be lower and lower until the prices are at parity.