So my cars been ****** up a lot lately lol, the RPMS have been all over the place, it has always done this, but got NOTICEABLY different after 2 days of raining(don't know if that had anything to do with it)

But i took it to chevy(i had no code thrown..it was just acting like it was running rich/lean aka no tuning lol) So they scanned the car, test drove it, and said it was my idle, that they had to relearn it? i guess that means tune it, so they did that, it didn't work, they actually made it much worse than it was before...also i threw a CEL after i took it home that day cause the GM dumbasses forgot to rehoook up my CAI and it was running straight from my chrome pipe(Threw a lean code) cleared it at autozone though.

They said that my MAF was not picking up? What does this mean? They said they're ordering another i'm just curious as to what made it go wrong or what happened to it...car has under 5 k miles...
any help appreciated, thanks