pub fn expect(self, msg: &str) -> T
So that says it's a method (its first parameter is the type itself, but named self rather than as a normal parameter so we can use method syntax instead of calling the function Option::expect) but it also takes an immutable reference to a string slice.
That second parameter, msg, is the text for a diagnostic if/ when you're wrong.
So, in a sense it's like value() but the diagnostic text means, when I was wrong...
let goose = found.expect("Our goose finder should always find a goose");