The Big Picture

The 3 (+1) "places" you need to keep in your head:

Working Directory  →  Staging Area (Index)  →  Local Repo (.git)  →  Remote (GitLab)
   (your files,          (what goes into          (the commit           (the "server",
    what you              the next commit)         history on            e.g. GitLab)
    see)                                           your machine)

Place	What it is	How SourceTree shows it
Working Directory	Your files as you see/edit them right now	"Unstaged files"
Staging Area (Index)	The "list" of what goes into the next commit	"Staged files"
Local Repository	The commit history, stored locally in the .git folder	The graph in the main window
Remote Repository	The copy on GitLab (server)	REMOTES in the left menu

The basic workflow is:

Change files → Stage → Commit (locally) → Push (to GitLab) → (someone else does a) Pull