Generally, when you write code you immediately own the copyright. Rights under copyright vary from place to place but in general is the exclusive legal right to control the rules for copying, modifying, and distributing this work.
You can permit other people to copy or modify your work. This permission is called a “license”.
Software licenses can contain pretty much anything, in theory. But what makes something Free Software is the concept of “copyleft”. Playing on the idea that copyright was historically used to withhold permissions (to copy, modify, or distribute software), “copyleft” uses copyright law to require these permissions be granted.
GNOME and its related technologies are licensed under free terms, most often a “copyleft” style license such as the GPLv2, or LGPLv2.1.
Please review the license of the software when making a contribution and make sure that you understand and agree with those terms. Submitting a patch certifies that you have done so and that you are not submitting someone else's code without permission. Our license choices embody our commitment to freedom and any contribution to our code base must be made with that in mind.