Rollover is the process of converting from a near-expiring front-month contract to a contract in a later month, i.e. carrying forward your futures positions. This entails closing your position in the contract that is about to expire and creating a new position in a contract that is further out in time.

Any future or option you buy will have an expiration date (last day until which you can trade that contract). So, for example, you can only trade the Nifty 22nd February future until February 22nd, 2018. What if you want to hold nifty future to March?
In this situation, you must withdraw the Nifty February future and enter a new position in the March future, which will be good until March 29, 2018. Rolling over is the process of shifting your position from one month to the next. This rollover can be completed at any time until the market closes on February 22nd, 2018.

So, if you bought the Nifty February future at 11050 and the Nifty future at 11000 on February 20th, you decide to roll over your position to March in order to continue your nifty future buy position. So, you'll simply sell the Nifty February future and buy the Nifty March future, which you can now hold until March 29th.

Note that you must pay brokerage and costs when you sell the August futures and must pay brokerage and charges again when you buy the September futures. The fees are identical to those associated with a typical buy-and-sell transaction.

Rollover of contracts during the embargo period is not permitted, according to the SEBI circular and exchange clarifications. You will only be allowed to exit an existing position if you have a position in a contract that is currently banned.