If you don't have your cron enabled, that wouldn't be a big deal. It probably means that someone has purchased something from your store and has caused the cache to become invalidated.
This would mean that if you're using a configurable product or something similar like a grouped/bundle product, your product view/detail page might show a combination that might not be available anymore. Customers would see it but not be able to add it to the cart. It would then drop off once you refreshed the cache.
edit: This happens if something gets sold out in any combination.
