What exactly does "thing : Thing" do?

Help and Feedback Scripting Support
#1

What does this even do?

function example(String : string) -- what does the colon in "String : string" actually do?
    print(string)
end

example("E")

Answer appreciated.

#2

It’s a Luau exclusive referred to as “export”.

#3

It’s typechecking.
It goes as following
variable name: type

Here’s a handy link elaborating on it.