artificialflow Cross compatibility means that it is compatible with the mentioned extensions enabled at the same time without causing problems or combining their features, regarding the license there are two, the one for the extension which is the implementation of the software (MIT, nothing to worry about) and another one for Fancybox, the software itself, managed by fancyapps.
You can install the extension to test it and see if it fits your requirements as mentioned here (https://fancyapps.com/pricing/), after that you should buy a fancybox license and keep it personally (there is no special installation or place to put the license, it just gives you the right to use it if you are claimed at some point).