Superheat & subcooling

That’s a broad question, in general too low sH will harm the compressor. Too high will as well yet your ac will be lame, not enough juice
Sub cool- too little. Not enough juice in the system and your gas may flash in the line before it reaches your Evap. Too high means too much juice in the system

You know the basic definition of super heat and sub cool? Just need gauges and a way to measured the temp of the lines. Most every tech uses digital gauges these days, they have K couple clamps and do the math for you