A tracking branch is a local branch with a remote counterpart it follows. Once set, git push and git pull need no arguments.
Set tracking on first push:
git push -u origin feature/auth
That -u is shorthand for --set-upstream. Now feature/auth tracks origin/feature/auth.
Inspect: git branch -vv shows tracking with [origin/feature/auth: ahead 2].
Change later:
git branch --set-upstream-to=origin/main main
Unset:
git branch --unset-upstream
Default behavior is configured by push.autoSetupRemote (new in Git 2.37) — set it to true and -u becomes implicit.