A for loop within a for loop is called a nested for loop. The outer for loop will assign a value from range to i, and the inner for loop will assign a value from range to j, and both values will be printed in the inner for loop:

for (i in 1..3) {
    for (j in 1..3) {
        println("$i , $j")
    }
}

Notice that on each iteration of the outer for loop, the inner for loop will be executed three times.