Its a mix of math and circuit analysis. I learned it engineering school because i graduated electronics engineering. She was talking about converting DC current (usually found in batteries) to AC current (high power like the 220 or 110 Voltage found in outlets) and vice versa. She was probably trying to create a battery using mana. High power AC mana to a DC battery mana source like the crystal thingy she showed her teacher.
AC = Alternating current : high voltage
DC = Direct Current : lower voltage
my bad she was converting DC to AC, the step down/step up part was that you can change the voltage of electricity by stepping it up or down through some sort of converter circuit. The phase and sine wave part is about alternating current moves like a sine wave or sinusoidal waveform
in short she was talking about electricity so dont worry if you dont get it

Honestly I’m enjoying the story but I don’t at all understand the math!!! Also I know most of these times of stories have a bunch of cliches but I kind of want the ending that is supposed to happen, I know that sounds terrible but still I don’t want just a random plot that makes her live past the age of 21, and if she does, I want it to make sense