Generator functions behave like regular functions in all respects, except for one difference. Instead of collecting results and returning them at once, they are automatically turned into iterators that yield results one at a time when you call next on them. Generator functions are automatically turned into their own iterators by Python.
This is all very theoretical so, let's make it clear why such a mechanism is so powerful, and then let's see an example.
Say I asked you to count out loud from 1 to 1,000,000. You start, and at some point I ask you to stop. After some time, I ask you to resume. At this point, what is the minimum information you need to be able to resume correctly? Well, you need to remember the last number ...
