"Full source" probably means that you can build the operating system but that it is not open source. If it was open source, they would use that term instead of "full source".
You have to look at the license for the operating system to see if it is open source. If it is not open source, there will be restrictions on what you can do with the source code of the operating system.
The Open Source Initiative has a good definition of "open source": http://opensource.org/osd
There are a bunch of details, but the summary is:
Open source software is software that can be freely used, changed, and shared (in modified or unmodified form) by anyone. Open source software is made by many people, and distributed under licenses that comply with the Open Source Definition.