The frontend of a web app includes the user interface through which the user interacts with the app. It is generally built using CSS, HTML, and JavaScript technologies. As it is the face of any web application therefore it has to be attractive, user friendly, and efficient.
The backend of the web app refers to the server side that includes the database and/or the coding. The backend of any web app makes the application function properly and efficiently.
Full Stack refers to both frontend and backend together. Full Stack developers work with both the frontend and backend of a website. They are familiar with HTML, CSS, JavaScript, and one or more backend programming languages.