What does “.()” mean in Kotlin?

A function that takes in nothing and returns nothing in Kotlin looks like:

var function : () -> Unit

The difference is that the function in your code takes in nothing, returns nothing, but is invoked on an object.

For example,

class Builder (val multiplier: Int) {
    
    fun invokeStuff(action: (Builder.() -> Unit)) {
        this.action()
    }
    
    fun multiply(value: Int) : Int {
        return value * multiplier
    }
}

The important bit here is the way we’ve declared the type of action

action: (Builder.() -> Unit)

This is a function that returns nothing, takes in nothing but is invoked on an object of type Builder.

This means when we use this builder like so

var builder = Builder(10)
builder.invokeStuff({
    var result = multiply(1)
    println(result)
})

The context of this has been set to the builder object and we can invoke functions declared within the builder.

Refer more here.