The explanation that is given by Maynard is correct, but the industry term is "calendar rollover".

The calendar rollover is a transaction where a futures contract is exchanged to another one, which has a further delivery date. For example, you had a position in September futures, during the delivery month (September) you can "roll your position" to December futures. It would involve two steps, first, you would need to offset your original position (if you were long, you sell, if you were short, you buy) then take the new position in the new futures contract.

This kind of transaction is very common in futures trading, as the quoted text says it, too. It is used a lot in hedging, when you have to set up a hedge (using futures) for a longer period, then the length of the futures contract that can be used. In that case, the hedger must roll the hedge forward (do a calendar roll on the futures positions he has).