The Mondschein Engine is an open-source software and it is completely free, where free is seen in the
sense of freedom (free will), not price (free beer). That means you may use, modify and distribute the
source code of the Mondschein Engine, with or without modifications at your own will, subject to the
terms of the GNU General Public License version 3 (GPLv3), or (at your option) any later
In short words that means that you are free to use the software in any (legal) way you want, with or without modifications, but you also must grant these rights to other users. Thus you must license any modifications you make on the original source code under the GPLv3 or a later version of the GPL. I will not discuss here in detail under which circumstances you must relicense your code under the GPL, this is discussed within the GPL itself and on the site of its authors, the Free Software Foundation (FSF).
The documentation is generally licensed under GNU Free Document License version 1.3 (GFDLv1.3) or (at your option) any later version.
(Short GFDL description)