This means that you can pay upfront for the next period. To give you an example:
Today is the 23rd of January and my account expires on the 2nd of February. But on that day I won’t be able to extend my account. Therefore I want to be able to extend my account today (23th of Jan) instead of waiting for the exact date of ‘required’ renewal. This way your account (and therefore your institution) will never be expired.