(... and why am I clearing it?)

The short version: 

Your internet browser saves files and settings as you browse the internet. These files and settings help improve your internet experience. Sometimes, they do the opposite, and make your experience worse, or even break it.  Sometimes we (or someone else) may recommend you clear your browser cache in order to resolve a technical issue.

Things to consider:

• Do only you use your computer, or did someone else use it recently as well?
• Have usual solutions not worked, like re-starting your computer?
• Internet Connection is fine for others?

What happens after you clear cache and cookies:

• Some settings on sites get deleted. For example, if you were signed in, you’ll need to sign in again.
• Some sites can seem slower because content, like images, needs to load again.

Bottom line - If you're reading this, there's a decent chance it is a good thing for you to try.

More info/Long Version: https://support.google.com/accounts/answer/32050