What is the "Free software only" option when installing Ubuntu?
FSF free is not the same as the free software only. There is free as in free beer and free as in freedom; FSF is free as in freedom, and the Ubuntu free only is free as in free beer.
FSF maintains a list of what they consider free distros.
See also Linux Libre.
Alas, many of those are great distros, but many are poorly maintained.
Of the distros listed, IMO the most actively maintained are:
Trisquel, which is based on Ubuntu, screenshot:
Parabola, based on Arch linux.
The others, including Musix and dynbolic, are great distros, but have not had recent releases.
There is also an apt repository for the libre kernel.
Keep in mind, although the beer is free, you should tip the bartender and support projects you use with donations - Ubuntu, Trisquel, or otherwise.
Ubuntu is a free software. In fact, Ubuntu is "The world's most popular free OS". But there are applications for Ubuntu which are not free. These applications are in generally included in restricted and multiverse repositories.
So, that option about you asked, Free software only make that these repositories (restricted and multiverse) to be disabled from your sources list. Like this when you will search something in Ubuntu Software Center for example, you will find free software only.
This allows to install a system without anything from the restricted repositories.
When you select this boot option components/software will be removed from the system after the file copy process has taken place.