What is a 1 in 40 fall mean on drainage pipe?

Oh, dude, a 1 in 40 fall on a drainage pipe means that for every 40 units of horizontal distance, the pipe will drop by 1 unit vertically. It's like the pipe is playing a little game of "let's go downhill slowly" as it carries all that water away. So yeah, it's just a fancy way of saying the pipe is sloping down to help with the drainage.