Overview
Moonfish is a free UCI chess engine derived from Stockfish, which is derived from Glaurung 2.1. For further information see (https://stockfishchess.org) and (https://github.com/official-stockfish/Stockfish).
Terms of use
Moonfish is free and distributed under the GNU General Public License version 3 (GPL v3). Essentially, this means that you are free to do almost exactly what you want with the program, including distributing it among your friends, making it available for download from your web site, selling it (either by itself or as part of some bigger software package), or using it as the starting point for a software project of your own.
The only real limitation is that whenever you distribute Moonfish in some way, you must always include the full source code or a pointer to where the source code can be found. If you make any changes to the source code these changes must also be made available under the GPL.
For full details, read the copy of the GPL v3 found in the file named Copying.txt.