I see some linux page says how to install the key:
wget http://packages.ros.org/ros.key -O - | sudo apt-key add -
What is "-O -" in wget?
It seems related with "add -".
How these works?
When you chain both commands you don't have to bother with saving a file and usually the command is shorter.
I used ** to indicate it is important.
this explains your command.