Urubu is licensed under the GNU Affero General Public License.
The first thing to understand is that this license applies only to the Urubu software. It has no implications on website projects managed with Urubu. You are free to license your projects in any way you choose.
The license only becomes relevant when you release a modified version of the Urubu software to the public in some way. In that case, you have to release the modified source code also, under the same license.
With the original GPL, a release meant shipping software to user that would install and run it locally. However, these days software is often made available by running it on a server. Under the original GPL, there would be no requirement to release the modified source code in such as case. This clearly violates the spirit of the GNU Licenses.
The GNU Affero General Public License fixes that problem. If you make the modified software available by running it on a server, you have to make the modified source code available to the users under the same license.