P is a node that extracts the xyz coordinates of each point in space. By having the x and z componenets set to 0, we're just finding out how far up the y axis each point is (i.e. how far above the ground it is). We want each blend to start at a certain height and finish at a certain height, but the blend input needs to be between 0 and 1, so if we want the blend to happen between height 2 and height 3 then we need to alter the "signal" coming out of P in some way so that 2 looks like 0 and 3 looks like one. The easy way to do that is to subtract 2 from the signal. However, these leaves us with a problem - There are still a lot of values of the signal outside the 0 to 1 range. That's easy to fix with a clamp function. Clamp just sets anything below 0 to 0 and anything above 1 to 1, but leaves values between 0 and 1 unaltered. We could probably get away without having the clams at all because the blender node is happy enough to interpret negative values as 0 and values about 1 as 1. By changing the values in the add function, you can adjust the height and the blend width for each transition.

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