Basically, the web site code comes in these three flavours. Each works together to bring websites to the end user. This code triple arrives at the web browser and the web browser in turn creates the website for the end user. Commonly know as ‘the front end’.
An artform unto itself, front end development is built up out of a combination of html, CSS, and javascript.
Often times these three are extended out, for example with javascript libraries, such as REACT, BootStrap, Node,js, three.js , etc . Really is a fantastic area of development and there are plenty of tutorials out there to get more familiar and practice with.
Eventually, as communication occurs between the web browser, interactivity throughout the front end, bounces information and events back and forth to the back end.