[git] 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 ?

This is no longer up-to-date!

Push.default is unset; its implicit value has changed in
Git 2.0 from 'matching' to 'simple'. To squelch this message
and maintain the traditional behavior, use:

  git config --global push.default matching

To squelch this message and adopt the new behavior now, use:

  git config --global push.default simple

When push.default is set to 'matching', git will push local branches
to the remote branches that already exist with the same name.

Since Git 2.0, Git defaults to the more conservative 'simple'
behavior, which only pushes the current branch to the corresponding
remote branch that 'git pull' uses to update the current branch.

When you push a new branch the first time use: >git push -u origin

After that, you can just type a shorter command: >git push

The first-time -u option created a persistent upstream tracking branch with your local branch.