As web development evolved, it separated into logical specialization: front end web development and back end development. While back end development involves the server-side development, front end is the final rendering.
Back end development (code that is executed on the server before sending to the web browser) includes database development, logins, and other “dynamic” functionality.
The front end development is the final rendering of the output for the devices and users. A back end developer might complete their work with basic/raw styling of the page, and the front end developer could take that output and add all of the treatment that make the page usable, appealing, and cross-browser/device compliant.
In practice, these teams work together and there is often high collaboration necessary to deliver a cohesive and usable final product.