mom wrote
What is set up ???
It's not a precise term. There are various adjustments that can be made on a guitar. Exactly what can be adjusted varies according to the type of guitar (EG you do not have the exact same adjustments on a classical guitar as you will have on a solid body electric).
One aim of this (already alluded to by Singemonkey) is to get a guitar where the action (the distance you have to push the strings down to fret them) is not too high - which means extra effort - or too low - which would usually result in buzzes. Another thing you'd be looking at is the intonation - how well the guitar stays in tune as you move from fret to fret.
But no need worrying about that now. Really. Do not lie in bed tonight wondering about this stuff. That's an order ?. Let the teacher check the guitar out when your kid goes for his first lesson.