ماذا يعني تعريف كائن كثابت في kotlin؟

class Car {
    var brand = ""
    var model = ""
    var year = 0
}

Class Main {
    fun main(args:Array<String>) {
        val c1 = Car()

        c1.brand = "Ford"
        c1.model = "Mustang"
        c1.year = 1969

        println(c1.brand)   
        println(c1.model)  
        println(c1.year)    
    }
}

var c1 = Car()