PianoRay is licensed under GNU GPL v3. See LICENSE for the full license text.
You are free to use the software however you want.
If you are using the code itself, i.e. using some code from PianoRay for your own projects, you must license the complete derived work with a compatible license.
If you are just using the output i.e. the rendered videos, you may use the output however you want, with or without credit.