What does Rollover mean and what are the charges involved?
Rollover is basically switching from the front-month contract that is close to expiration to another contract in a further-out month i.e carrying forward of your futures positions. What this means is closing your position in the contract which is about to expire and opening a similar new position in a further-out month contract.
When you buy any future or option, it will have an expiry day (last day until which you can trade that contract). So, for example, Nifty 22nd Feb future, you can trade only until 22nd Feb 2018. What if you want to hold nifty future to March?
In such a case you have to exit Nifty February future and take a fresh position in the March future which would be then valid till, in this case, 29th March 2018. This process of moving your position from one month to another is called as rolling over. This rolling over can be done anytime before the market closing on 22nd Feb 2018.
So for example, if you have bought Nifty February future at 11050 and on 20th February assume Nifty future is 11000, you now decide to roll over your position to March since you want to continue your nifty future buy position. So basically what you will do is sell the Nifty February future and buy the Nifty March future which you can now hold until 29th March.
Note: When you sell the August future you have to pay brokerage & charges and when you buy back the September futures you have to again pay brokerage & charges. Charges are similar to a normal buy and sell transaction.