It is an open-source, HTTP web-server most widely available on the internet which supports multiple languages and modules like Perl, PHP, Python, CGI, and the other default Linux server applications. Whenever you request for a website, apache is the application that services this request (on a Linux server)Apache handlers allow you to control.
What does Apache do with certain file types?
When Apache sees a file, it has an action built-in for that file type and will perform that action. If you wish Apache to do a different action, you will need to make a handler to tell Apache to perform that action. For example, if you use a file type that requires a special service to run it, such as a file with server-side includes that is not named with a .shtml extension, you need to tell Apache to treat these files differently.