What does mod_gil_not_used do?

[sambaPython24]

Hey, thank you for the work that you are putting into this package.

When I was using the introductary examples, I saw that there was the element

mod_gil_not_used in 
PYBIND11_MODULE(example, m, py::mod_gil_not_used()) {

Why should I use this option and what implications does it have?