What does git push -u mean?

I have two different versions of git.In the 1.6.2 version, git push does not have the -u option. It only appears in the 1.7.x version.

From the docs, the -u is related to the variable

branch.<name>.merge

in git config. This variable is described below:

Defines, together with branch.<name>.remote, the upstream branch for the given branch. It tells git fetch/git pull which branch to merge.

What is an upstream branch ?