Projectile is a emacs package for juggling multiple projects in emacs.

Think of a project as a git repository. You configure it to search for new project in a path; on my systems it is in ~/Code, and then you give it a shortcut; mine i M-p. Then you can for example git M-p p to switch project or M-p SPC to search for a file in the current project. Projectile can do a lot of other stuff as well, and there are many different other stuff that integrate into it.