A stored property conforming to a Protocol with a method in it..?

For this given example (on the video: protocols as Types and Apple Documentation):

_Code Begins Here_

protocol RandomNumberGenerator { func random() -> Double }

class LinearCongruentialGenerator: RandomNumberGenerator { var lastRandom = 42.0 let m = 129968.0 let a = 3877.0 let c = 29573.0

func random() -> Double {
    lastRandom = ((lastRandom * a + c) % m)
    return lastRandom/m
}

}

class Dice { let sides: Int let generator: RandomNumberGenerator

init(sides: Int, generator: RandomNumberGenerator) {
    self.sides = sides
    self.generator = generator
}

func roll() -> Int {
    return Int(generator.random() * Double(sides)) + 1
}

}

var d6 = Dice(sides: 6, generator:LinearCongruentialGenerator())

_Code Ends Here_

We see that in class Dice there is a stored property called: generator.

generator is of type RandomNumberGenerator which is a Protocol defined at the top.

How come generator can conform to this protocol? To conform with that protocol, shouldn't an object needs to have the random() method?

I understand Classes conforming to a Protocol, but how come this variable be a type of RandomNumberGenerator?

This also means that I haven't understood this video's topic so it's a more general question about Protocols as Types..

I will appreciate your helps, and thanks in advance! Mert