I'm thinking about just getting this chromatic tuner but i dont know what the + or -20 means, is it a bit complicated for what im going to use it for?

my jackson is getting delivered today and im pretty sure it'll need a setup, but i usually just tune by ear via youtube video

ive been using my sister's qwik tuner, think its decent enough to get a proper setup done?
your sisters will work in a pinch... but a good chromatic tuner is a good start...
2003 Fender Standard Strat w/ Texas Specials
2012 Babicz Identity Dreadnaught
2015 Gibson Les Paul Traditional SR
Line 6 POD HD500
Peavey XXX 112
Fender Blues Jr
Chromatics are the best.. but I mean, any tuner will work as long as it's accurate.

How do you say "I'm okay" to an answering machine?

Chromatic just means it can read all the notes vs. just the ones in std tuning. There are usually flat and sharp buttons on non-chromatic tuners that allow you to get the other notes as well. The +/-20 is probably a cents figure, and is horrible if that's the accuracy. You want +/- 1 cent.
As fly135 said, you want a tuner with an dead-on accuracy. It's intonation we're talking about after all.
i would just get a korg. i used one when i started lost it then used one like the qwik for bout a year and finally got a new korg to do my intonation with. they're like 10-20 bucks and really useful. also i like to keep a small pedal cable attached to it all the time so tuning is more accurate.