This is the most commonly used function in Kotlin. The filter function takes one lambda expression, which takes one variable as an input and returns the same variable as a result. The filter function applies the lambda expression and returns a list as a result. Let's create a list of integers and apply some filters:

var numbers = listOf<Int>(1,2,3,4,5,6,7,8,9,10)

We can filter out all elements that are greater than five as follows:

var newList = numbers.filter{ i -> i > 5 }
println("Filter out greater than 5")
println(newList)

i represents the element in the list and i > 5 is a condition that filters the results. The lambda expression filters the elements and the filter function returns a new list. 

We can filter out all elements that are smaller than or equal to five as follows:

newList = numbers.filter { i -> i <= 5 }