A cache is a place to store or hide something. When your browser displays a web page, it stores a copy of the page in its cache on your machine. The next time you visit the page, the browser scans the files in its cache and if it finds the page, it will display the copy stored on your machine instead of going to the web to retrieve a fresh copy. This reduces the time you wait for the page to load because loading the copy on your machine is quicker than retrieving a new copy from the web site.
The browser does need to contact the web site every now and then to find out if the page on your machine has become out-dated. How often the browser checks the web site to see if the page has been updated is something you configure within the browser.