The phrase "slam dunk" is an idiomatic expression that means a clear, unambiguous success or victory. It originated in basketball, where it refers to a shot that is so easy to make that it seems like a guaranteed score - literally, the ball just needs to be thrown up into the hoop and it will go in without any effort needed. The phrase has since been adopted more broadly to refer to anything that is certain or assured, as if it were an automatic success.