Suppose you have multiple threads which don't really touch each other's data. Those should execute as independently as possible. If you have a "global lock" which you need to acquire in order to (say) call a function, that can end up as a bottleneck. You can wind up not getting much benefit from having multiple threads in the first place.
To put it into a real world analogy: imagine 100 developers working at a company with only a single coffee mug. Most of the developers would spend their time waiting for coffee instead of coding.
None of this is Python-specific - I don't know the details of what Python needed a GIL for in the first place. However, hopefully it's given you a better idea of the general concept.