The first thing a browser does when loading a web page is to check for that the page's text and graphics files are in the browser's cache folder on your hard drive. If the text and graphics for that page are not found, the browser stores all the text and graphics in the cache folder. Then when you revisit that page, it can just read the files from your hard drive rather than downloading them again off the network, which is usually much faster.
If you have your cache set to a large number, you can store a lot of files on your computer without really realizing it. The more files on your computer there are, the harder it may have to work to do some things. If your computer is running unusually slowly, you might try emptying the browser's cache to speed things up.