Modems are even slower. When you go to a web page to download its text and images, it can take a long time using a modem. We know that most web pages don't change much. It would be nice if we didn't have to pull all that data across the network and down through the modem every time we want to view a page. So each time we read a page, the page and all its graphics and other files are stored nearby, in a cache, in case we go there again in the near future.
Different browsers have different settings for their cache.
It also lets you control how much disk to use before bumping old stuff. You should select an amount of disk equal to the space taken by all the places you go regularly. If you always seem to be waiting for the network, your cache may be too small. You may not be keeping pages long enough, so that when you revisit them they're no longer on your hard disk for quick access. They're getting bumped by new stuff.