A rollover is the process of extending an open position past its original expiry or settlement date. It keeps the same market exposure on the books instead of letting the position close out.
For a dated contract, a rollover means closing the expiring contract and opening a later-dated one at the same time, so exposure carries forward. This is routine in futures, forwards, and options, where traders roll before expiry to avoid delivery or cash settlement of the expiring contract.
In forex and CFD trading, rollover has a second meaning: the overnight financing adjustment applied when a position is held past the broker's daily rollover time. That adjustment is a swap charge or a swap credit, set by the trade direction and the interest-rate difference between the two sides. The contract-roll meaning moves exposure to a new expiry; the financing meaning is a daily cost or credit on a position that stays open.
You hold a crude oil futures contract that expires in June.
You want to keep your oil exposure, so you close the June contract and open the July contract.
That switch from the June contract to the July contract is a rollover: it moves the position into a later expiry month without giving up exposure.