What does 'the URL is local' mean?

Question

I am debugging some code and there is this check (in an ASP.NET MVC controller)

if (Url.IsLocalUrl(returnUrl))

So I check the documentation and it says Returns a value that indicates whether the URL is local.

But what does that mean, 'the URL is local'?

If I hit a webserver, when does the webserver say 'the URL is local' ?

Answer 1

In the ASP.NET MVC blog Preventing Open Redirection Attacks (C#) you can find an explanation of why you should use it, but, as is tradition with MVC's documentation, it's not explained how it works.

You can read that from the source presented there though: it checks whether an URL starts with / or ~/, meaning: whether it is a relative URL which thereby points to the same domain.