What is "generators"

Of Wikipedia:

A generator is a special routine that can be used to control the behavior of an iteration circuit. A generator is very similar to a function that returns a matrix, in which a generator has parameters, can be called, and generates a sequence of values. However, instead of building an array containing all the values and returning them all at once, a generator produces the values one at a time, which requires less memory and allows the caller to start processing the first values immediately. In short, a generator looks like a function, but behaves like an iterator.

Generators can be implemented in terms of more expressive control flow constructions, such as co-routines or first-class continuations.