[WARNING, not an expert, but what I believe I know.]
Perris, those are Debian's guidelines for including software in their distribution. They are very an4l [per LofLA] about licensing, ala their recent licensing issues with Firefox, they forked into IceWeasel [stupid].
As far as Gnu Public License software [GPL] is concerned, that is where licensing comes into play. If you modify a GPL'd piece of software you have to, by law, provide the source code because that is how the license is written.
There are also some legalese that comes along with redistributing software that includes shared libraries that are released under the GPL. E.g. if you write some software and in order to run it, you need some GPL'd libraries, if you want to statically link and distribute those libraries along with your program, you must release the source code as well. There are notable exceptions to this, including the libstdc++ libraries which are GPL, but do not require you to release your source if you statically link to it.
The other main open source license is the BSD license which allows you to create proprietary works from released source code as long as you acknowledge the source. The most common example of this is the networking stack built into Windows XP, which is BSD licensed.
Basically if you write software derived from GPL'd software, it must be GPL'd itself, if you write software derived from BSD licensed software you must acknowledge the author.