Your browser's cache (pronounced "cash") stores information to help pages load faster. Cookies are small files stored on your computer that save specific information (like passwords and site-specific preferences). If the data stored in the cache and cookies is old, it can prevent sites from loading correctly and clearing them can help resolve these issues.
This article links to browser-specific documentation/sites for best service, as many browsers are updated frequently and instructions may change.
Press the listed keyboard keys together at the same time, then let go. This needs to be done with the browser selected.