Arbitrage is the practice of finding and taking advantage of price discrepancies on the same asset across different markets. This usually happens when global price feeds are not aligned and the consumers are just paying the written face value for the asset.
For example,
cryptocurrency prices on Korean exchanges can be different from those on US exchanges. An arbitrage trader would be in both markets in order to buy in one and sell in another for profit.
So, if at 7:00 pm Bitcoin is $20,000 on Binance and $16,000 on Coinbase, the arbitrage opportunity is $4,000