Interfaces question

I'm trying to understand the -why- of this ... and I'm really struggling to grasp the concept of what I'm telling the compiler to do when I use an IInterface syntax. Can anyone explain it in a "this is what's going on" way?

Anyway ... my main question is....

What is the difference between

public IEnumerable MyMethod() {...}

and

public string MyMethod() : IEnumerable {...}

Why would you use one over the other?

Thanks,
Simon

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