The first item is the object that you actually want to apply the constraint to. The second item is the object you want the constraint to be related to. Assuming you linked up the constraints correctly you shouldn't have to reverse the items at all. The multiplier is exactly what a multiplier should do.
The relationship "Less than or equal" means that the cloud's height can be smaller than 400, but not larger. This is helpful when you're adding other constraints that might conflict with each other.
Example: if the superview's height is the first item and the cloud's height is the second item, and the multiplier is 1, the superview's height will be equal to the cloud's height. If you reverse the items, the cloud's height will be equal to the superview's height.
Now, let's say the cloud's height is item 1 and the superview's height is item 2. If the superview's height is 1000, the cloud's height will be 1000. If you change the multiplier to 0.4, the cloud's height will be the superview's height * 0.4, so 1000 * 0.4 = 400.
If we then set the relation from equal to less than or equal, it will probably stay the same. If we then add a constraint; setting the cloud's height to be 500, we will get a conflict. If we change the constraint from 500 to 300, your cloud's height will be 300.
Hope this helps!
Edit: this link and this link should help you grasp some of the constraint features.