First you must consider what the job of the diode or a single rectifier is doing in the circuit. Think of it as a valve that which lets current flow in one direction but blocks in the other direction. Such as for example in a Human heart value, where it's job is to pass the flow of blood in one direction and block the reverse flow of it in the other. Forcing blood to go to the rest of the body in certain direction, or when a spray hose lets water flow when press. Remember input of the flow of power or current is changing, first in one direction then in the other as a full 360 degree's circle but the load only see half of it. You can think of the RMS as a tally of all grade average of the students in that hold class, and the other half is where other class does not show up. The main idea is changing AC (Alternating Current) to DC (direct current) toward the load. I hope this gives you a birds eye view of what is happening here in a rectifier. But you can be more exacted using mathematic.