Difference between `git branch --set-upstream-to=origin/<branch> mybranch` and `git remote add upstream URL` [duplicate]

Question

I have a repo in my account which is not forked from anywhere. But when I tried to do git pull in one of the branches, I got this message:

There is no tracking information for the current branch.
Please specify which branch you want to merge with.
See git-pull(1) for details.

    git pull <remote> <branch>

If you wish to set tracking information for this branch you can do so with:

    git branch --set-upstream-to=origin/<branch> mybranch

Doing git branch --set-upstream-to=origin/mybranch mybranch solved the problem.
But how is this different from git remote add upstream URL?

Any suggestion gratefully received. Thanks in advance.