Your browser's cache is a collection of web pages, including the text, images, and most other media contained on them, that is stored on your hard drive.
Having a local copy of a web page makes for very quick loading on your next visit because your computer doesn't have to download from the Internet all of that same information again.
If browser caching makes pages load faster, why would you ever want to clear your cache? A few reasons come to mind.
Firstly, clearing your cache forces your browser to retrieve the newest copy available from the website, something that should happen automatically but sometimes does not.
You might also want to clear the cache if you're experiencing issues like 404 errors or 502 errors (among others), sometimes indications that your browser's cache is corrupted.
The attached document contains instructions for clearing the cache for major browsers supported by McGraw-Hill products. Both windows and Mac instructions are available.