Before you can see a web page you need a client (browser) that contacts a web server. The interaction is established via a protocol named http or https. This protocol sends a REQUEST and a web server returns a RESPONSE.
When a web server response is the content of html, css and javascript without any other transformation before hand, we call that a STATIC web page. A DYNAMIC web page is when a web server response is comprised of data rendered after the request. For a dynamic site the web server will contact other internal services that will provide the data necessary to send a response converted into httml, css and javascript.
The development and design work involved into displaying a web page is called FRONT-END. The development of services that provides data and content beyond html, css and javascript is called BACK-END.