The Python Global Interpreter Lock or GIL, in simple words, is a mutex (or a lock) that allows only one thread to hold the control of the Python interpreter. All the GIL does is make sure only one thread is executing Python code at a time; control still switches between threads. What the GIL prevents then, is making use of more than one CPU core or separate CPUs to run threads in parallel.
Python threading is great for creating a responsive GUI, or for handling multiple short web requests where I/O is the bottleneck more than the Python code. It is not suitable for parallelizing computationally intensive Python code, stick to the multiprocessing module for such tasks or delegate to a dedicated external library. For actual parallelization in Python, you should use the multiprocessing module to fork multiple processes that execute in parallel (due to the global interpreter lock, Python threads provide interleaving, but they are in fact executed serially, not in parallel, and are only useful when interleaving I/O operations). However, threading is still an appropriate model if you want to run multiple I/O-bound tasks simultaneously.