Covariance

Covariance is a concept in object oriented programming. That's all you need to know for the context of this post.

Here is a quote from an article where the author attempts to explain a type of covariance. If you weren't already confused...

Covariant return types on method overrides make this explicit, allowing the use of a type that is derived from the return type of the method being overridden as the return type of the method doing the overriding.

What a mouthful!