Height map is a VERY IMPORTANT MECHANIC that you want to fully understand so pay close attention.
Height map is the hardest of the three to explain and understand so you might have to re read this section multiple times to fully understand it. Height map as I said earlier is the highest block at certain cordinates in the world. The reason this is important in mob farms is because a mob spawn attempt uses height map to pick its y elevation. If the height map goes from -64 to 100 then there are 165 possible positions to choose. In other words if you have one spawning platform at one level the chance of a sucessfll spawn is one in 165. If the height map goes from -64 to -63 than the spawn chance is 1 in 3. So the lower in the world your farm is the higher the spawn rates. But if you have blocks above your farm it still reduces the spawn rates by the same ammount. So if your friend ever builds a mob farm on a multiplayer and you want to sabotage it you could just place glass or string at world height above it and it would become 383 times slower > : ).