use of if-let

Here is my code:

func isDivisible (#dividend:Int, #divisor: Int) -> Bool? {

if dividend % divisor == 0 {
    return true
}

return nil

}

if isDivisible(dividend: 4, divisor: 2) == true { println("Divisible")}

else {println("Not Divisible")}

this works to solve the problem... in the answer portion of the video, they used the following code to call the function:

if let culprit = isDivisible(dividend: 4, divisor: 2) { println("Divisible") } else { println("Not Divisible") }

I was confused because I could not figure out the significance of the "if let" keyword. if you take out let, i get an error. Does if let basically mean that as long as this is not nil, execute the code that follows?