this

As with Java, Kotlin uses this as a pseudo-property to refer to the instance of the object in whose function you happen to be running.

You can use that as a prefix for property references or function calls, such as this.count:

class Foo {
  var count = 0

  fun something() {
    this.count += 1
    println("something() was called $count times")
  }
}

fun main() {
  val foo = Foo()

  foo.something()
  foo.something()
  foo.something()

  println("the final count was ${foo.count}")
}

Or you could use it on its own to refer to the current instance, such as for supplying that object as a parameter to a function:

class Foo {
  fun printMe() {
    println(this)
  }
}

fun main() {
  val foo = Foo()

  foo.printMe()
}

Occasionally, you will see odd variants of this, such as this@Foo. We will see what that syntax means later in the book.