An official-sounding definition is this: “A cache is a repository for stored data that is used to expedite the process of retrieving data… Internet browsers use caching to store web pages by storing a copy of visited pages and then using that copy to render when you re-visit that page.“

Basically your browser will store a version of your webpage and display that for you unless it thinks that there’s a new version of that page.  Sometimes it’s wrong about there not being a new version, so you see an out-of-date version of your page – ugh.  To fix it, you have to clear your cache.

As a website-owner you are going to encounter this problem more frequently then your website visitors.  This is because you are most often checking your website and often viewing a webpage that needs to be updated right before the update.  So your browser is more likely to still have the old version of that page stored then the newly updated version that most of your visitors will see.