Those functions that take another function as an argument or return a function as result are called high-order functions. Consider the following example to understand it better:

    fun highOrderFunc(a:Int, validityCheckFunc:(a:Int)->Boolean) {//(1) 
      if(validityCheckFunc(a)) {//(2) 
        println("a $a is Valid") 
      } else { 
        println("a $a is Invalid") 
      } 
    } 
 
    fun main(args: Array<String>) { 
      highOrderFun(12,{ a:Int -> a.isEven()})//(3) 
      highOrderFunc(19,{ a:Int -> a.isEven()}) 
    } 

In this program, we've declared a highOrderFunc function, which will take an Int and a validityCheckFunc(Int) function. We are calling the validityCheckFunc function inside the highOrderFunc function, to check whether the value was valid or not. However, we are defining the validityCheckFunc function at runtime, while we are calling the highOrderFunc function inside the main function.

Here is the output:

a 12 is Valid 
a 19 is Invalid