GitHub Desktop is an application that lets you perform different Git actions through the use of a GUI instead of the command line. You can use it to stage files, commit files, and create branches, among other things. It also has certain features that let you interact with GitHub, like pushing and pulling commits to your remote repo.
There are other Git GUI applications besides GitHub Desktop, like Sourcetree and GitKraken. They generally have similar features, but you may encounter some differences as well if you use a different application.
If you’re comfortable using Git on the command line, then it’s not necessary to learn how to use GitHub Desktop. However, if you find working with Git on the command line to be tedious, or if you’re still uncomfortable with using the command line in general, then GitHub Desktop can be a good tool to add to your arsenal.