An Arbitrage would mean a trade which would result in risk less profit.
Do note that on expiry of a future contract the price of the spot (equity) will be equal to the price of the future contract. In case you observe any difference, you can buy/sell equity and at the same time sell/buy futures and let it expire which will result in a risk less gain. However the chances of finding such opportunities are very miniscule.
For eg: If on the last trading day of May expiry,Reliance is trading at 1045 and if the May Futures of Reliance is at 1049, you can buy the stock and sell an equivalent amount of future contract to make a riskless gain of Rs.4/-