What does cd - (cd hyphen) do?

cd - switches between the old and new present working directories.

avinash@avinash:~$ cd -
bash: cd: OLDPWD not set
avinash@avinash:~$ cd ~/Desktop
avinash@avinash:~/Desktop$ pwd
/home/avinash/Desktop
avinash@avinash:~/Desktop$ cd -
/home/avinash
avinash@avinash:~$ 

See also,

avinash@avinash:~$ echo $OLDPWD

avinash@avinash:~$ cd ~/Desktop
avinash@avinash:~/Desktop$ echo $OLDPWD
/home/avinash
avinash@avinash:~/Desktop$ cd d
avinash@avinash:~/Desktop/d$ echo $OLDPWD
/home/avinash/Desktop
avinash@avinash:~/Desktop/d$ 

The $OLDPWD variable stores the path of the previous present working directory.

Avinash Raj's answer is completely correct but as for the manual entry, you can get the POSIX manual pages and then man cd will work:

sudo apt-get install manpages-posix
man cd

The bit that tells you all this is the OPERANDS section:

-      When a hyphen is used as the operand, this shall be equivalent to the command:

       cd "$OLDPWD" && pwd

which changes to the previous working directory and then writes its name.