Is let a get only variable?

Question

I'm a little bit confused with this.

We have variables in swift that can have a setter, getter or both and constants which can't change his own initial value,

So, is a var with only a getter the same as a let? If it is, what is the idea in a protocol to set a variable like this

protocol myProtocolA {
    var myString: String {get}
}

vs

protocol myProtocolB {
    let myString: String {get} //I don't think this will compile but I'm curious
}