In software engineerig, the terms front end and back end refers to the separation of concerns between the presentation layer(front end), and the data access layer (back end) of a piece of software.
source https://en.wikipedia.org/wiki/Front_and_back_ends
Are you asking what you should learn? If so it is up to you. Whether you should learn front-end or back-end programming. If so can't really help. It depends on whether you want to work with data management (back-end) or UI front-end.
