In India, stock and Index futures expires on last Thursday of the month. If that day is holiday then previous day i.e. Wednesday. Rollover is nothing but exiting from the current position and take fresh position on same stocks in another series.
e.g. If Mr. A has made a 1 future contract on Tata Motors for a price of Rs. 470 with expiry of such contract in August, 2016. However Mr. A is bullish on Tata Motors that price may further increase to Rs.480 in September, 2016 series. Then he may exit from the current position and take a fresh position for September series.