Please explain swift code.

I'm not sure which challenge you're on, but I'll take a crack at explaining this. Basically the variable i is short for index. We create it for the purpose of keeping track of how many times we've gone around the loop. So I've reworked your code a little bit:

let numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

var i = 0

while i < numbers.count {
    println(numbers[i])
    i += 1
}

So what I did is create a loop. It says as long as i is less than the number of items in the "numbers" array, it will keep going around the loop. Then, inside the loop, we use the variable i for another purpose. We use that number to put iside of the square brackets, which will find us a certain index in the array. So the first time in the loop, i is 0 so it will go to numbers[0], then i gets incremented. Next time around the loop it's numbers[1], and then i gets incremented again. So what this code does is go through each number in the numbers array and print it out.