1. RMS means root-mean-square. It really should be called "square-mean-root" as you square your function, average it and then square root it. Why do we need it? AC voltage is sinusoidally varying; its average value over a cycle is zero. That's not very useful. However, in power calculations, where P=V^2/R, an average of V^2 is just as good. This time, if we square the voltage, it's always positive and now an average makes sense. I won't go into the calculation details, but the RMS voltage is the peak voltage divided by sqrt 2, for a sinusoidal wave only (which is VCE physics). Just know how to calculate RMS voltages, currents, what they mean and the fact that P=I^2 R or V^2/R refers to RMS currents and voltages.

2. If a photodiode is forward-biased, it is a regular diode. Only when reverse-biased does its behaviour depend on light like in the course. Then, the photocurrent is directly proportional to the intensity of light striking the diode. If the light intensity doubles, the current doubles.

3. I'm not great with transistors, sorry. My electronics knowledge is strictly limited to what's in the course.