Many people have trouble getting the hang of dependency injection, at first. And I think part of the problem is that it is actually so simple that we're inclined to look for something more complicated. "Surely that there has to be more to it?!", so to say.
So, with that in mind, imagine that you're writing an app that gives weather reports. You need a cloud-service (excuse the pun ;) ) to provide the data, and at first you go for a free weather report provider, but in future you'd like to integrate a weather service with better accuracy and more features. So, as do all good object-oriented developers, you make a WeatherClient protocol and back it initially with an implementation based on the free, online data provider