This tool lists which web files on a website are cached and which are not. Furthermore it checks by which method these files are cached and what the expiry time of the cached files is.
This is the amount of minutes/hours/days the particular web file is set to be cached, also called the expiry time. After this specified time period the cached file will be refreshed with the latest version of the file.
This shows you the caching method that is being used for the particular file. The two most common types are "cache-control" and "expire".
When you see this button behind a file it means the etags are set for this file.
By using a browser caching mechanism you tell the browser of your visitor to copy and store your web files for later use. When your visitor re-visits your website these copies can be used without the visitor having to re-download the files. This can have a major effect on website speed, especially when you load a lot of external web files on your web pages.
Read this leverage browser caching guide to learn how to enable caching for your website.