reflections in transmission lines
Alright, Einc and Erefl are called "wave amplitudes". They are similar to voltages, but they "move" along the transmission line. What do I mean by that? Well, imagine you are on vacation at the beach, and you decide to go stand in the water. Well, the height of the surface of the water is always changing due to the waves. If you look far out, you can see a wave peak, and it is travelling towards you. When it hits you, the water height is a maximum.
So that would be like the Einc on a transmission line, some measureable thing that has an amplitude and is moving "towards" the load.
Now, if you were back on the beach, and decided to go stand in the water, but in front of a big concrete sea wall. You might notice that the waves come in from the sea, go past you, hit the sea wall, and reflect backward out to sea. If you watched the water from far out to sea, you could see the waves coming in. If you watched the sea walls, you could see the waves reflecting back.
But if you stared staight down at the water surface where you were standing, you would not see any waves at all, you would just see a surface height of the water. And although you could see that surface height changing, you would not really know what was making up the changing water height. In other words, you could measure the amplitude of the water height at that one point, and it would be a superposition of the incident waves AND the reflected waves at that one physical location/time instant.
same with microwaves. At the load you can measure the "height" of the waves (in volts if it is a wire transmission structure, or in some Efield intensity if in a waveguide). This "voltage" is the superposition of incident and reflected wave voltages, so it is Einc + Erefl.
It is important to note that the wave phases are very important. For instance, if the load is an open circuit, and the reflected wave is in phase with the incident wave, then the voltage measured at the load is 2 x the incident wave amplitude.