Upstream
In plain terms
The original or main repository a branch tracks.
Going deeper
Upstream refers to the remote repository or branch that your local branch is based on and pushes/pulls against. It's the source you sync with.
Under the hood
In forks, 'upstream' often names the original repo; tracking branches define push/pull targets.
Examples
Tracking branches, fork's origin, git push -u, syncing with main repo.
Think of it like...
Like the main river your tributary flows from and back into.