Cache Lifespan Setting

The guidance for Cache lifespan says “Time after which a new cached version will be generated (0 = only when the cache is emptied)”.

Does that mean (a) each individual cached page will be automatically regenerated after that period, (b) each individual cached page will be deleted after that period (so it can be regenerated when visited), or (c) the entire cache will be deleted after that period (so its pages can be regenerated when visited)?

Thanks.