Open Source
Terminology
Let's talk about Open Source in general, then specifically what this means regarding Scribus.
Open Source in its most restricted meaning simply means that the source code is available to review, though it may not be freely available. Some proprietary companies may "open" their source code to only a limited audience. Open Source does not necessarily mean you can use it, modify it, or even have a copy of it, so there may be a very restrictive license associated with that source.
When you combine Open Source with something like the GPL, such as Scribus does, you allow anyone to view, copy, modify, and use that source code in any way they see fit. The main restriction is the acknowledgement of the origin of that code, which is to remain with it whenever it is transmitted on from you.
Origins of Scribus
Although at this time the ongoing work of many, Scribus originally began with one person, Franz Schmid, making something useful for himself. Then he decided to make it useful for others to, after which the Scribus Development Team was born