What does 'cd -' stand for?

cd - brings you back to the last directory.

$ cd ~/Desktop
$ pwd
/Users/daknok/Desktop
$ cd /
$ pwd
/
$ cd -
$ pwd
/Users/daknok/Desktop

cd - returns to the previous directory you were in.

Say I'm in /usr/ and I type cd /var/local/someplace/else

Then I use cd - I'll return to /usr

cd - returns to the directory you were previously.

For instance:

marcelo@marcelo:~$ cd /opt
marcelo@marcelo:/opt$ cd /usr/bin
marcelo@marcelo:/usr/bin$ cd -
/opt
marcelo@marcelo:/opt$ 

I was in /opt, changed to /usr/bin, and then went back to /opt with cd -

If a single dash is specified as the argument, it will be replaced by the value of OLDPWD.

The OLDPWD is set by cd command and it is the previous working directory.