GIL is a promise from the internal implementation (eg CPython) of Python that things will happen atomically within the Python interpreter. This means that when multiple threads try to access and modify Python objects at the same time, the GIL ensures that only one thread can execute Python bytecode at any given moment, preventing potential conflicts and ensuring data integrity. However, this comes at the cost of limiting the full utilization of multiple CPU cores for certain CPU-bound tasks.

Non-GIL adds some complexity to the implementation and some risk when writing multithreaded code at the benefit of improving performance.