The licence you release your work under is important. A license is the legal side of things.
If you don't release your work under a compatible licence, we can't use it.
I don't know what the Creative Commons Attribution-Share Alike 3.0 actually says (you'd have to read it), but it probably basically comes down to this:
- You provide your work as well as the sources to this project.
- Anyone else can take your work from our project to use it for whatever they want, as long as they keep it under the same license.