Front end development deals with coding the client side of an app, the part the user sees and interacts with.
HTML, CSS, and Javascript are standard languages, but there are many design and graphic tools and libraries to know as part of the job.
A front end developer is responsible for building components according to spec, or may be more involved in design and add their own aesthetic. It depends on the team size and having preexisting designs.
Front end developers have to know about accessibility, responsive design, and browser compatibility.
Advanced CSS techniques using SASS compiling and BEM/SMACSS naming will keep your code organized and make sense to other developers.
Advanced Javascript developers can build custom application UIs and work with frameworks like React.