Trussrod

I'm completely new to guitars, period. I'm playing a cheap bass that was loaned to me by my music minister. He's said I can do with it whatever I want...so I was reading through some posts about intunation, and discovered that the guitar I'm playing is out of tune. I adjusted the saddles(I think) and brought it into tune...the 12th fret is in tune with the open string. I saw someone saying only a certified luthier should adjust the trussrod, because it could do serious damage to the neck. Anyone got a few minutes to explain some of the basic fundamentals of the guitar setup to me? Kinda/sorta what things are, what they do, why I would adjust them, etc. Basics.. thanks in advance!