Coroutines don't really have anything to do with threads. They are just a way for you to model suspending and resuming functions.

If you have every tried to implement a complicated iterator you will find that you model a for loop in your head but that the iteration of the loop is controlled from outside using the ++ operator. The ++ operator allows the iterator loop to go one more time round. Normally you have to manage this with state stored in some object. With coroutines you can actually write a loop and co_yield from the middle of the loop to implement the ++ operator.