Front-end development refers to the client-side (how a web page looks). Back-end development refers to the server-side (how a web page works).
Front-end code is used to create static websites, where the purpose is to display the web page. However, if you want to make your website dynamic (manage files and databases, add contact forms, control user-access, etc.), you need to learn a back-end programming language, like PHP or Python, and use SQL to communicate with databases.
If you already have some backend experience, you should know that you can use W3Schools Spaces to build, test and deploy backend code as well.