The front-end is the part of a site the user sees and interacts with (pages, buttons, forms); the back-end is the hidden part running on the server that processes data and makes everything work behind the scenes.
The front-end is everything that appears in the browser: text, images, menus, animations. It is built with HTML, CSS and JavaScript, often through a framework like React or Vue, and must be responsive and well crafted in its UX/UI.
The back-end lives on the server: it handles logic, user authentication, payments and communication with the database. When you submit a form or complete an order, it is the back-end that processes it and saves the data.
Front-end and back-end communicate through APIs: the front-end asks for data, the back-end responds. Someone who masters both is a "full-stack" developer. A solid project needs care on both layers: in our web development we work full-stack, from design to infrastructure.
Related terms: APIDatabaseFramework