Not an idiot. I'll assume we're talking adjustment for the rear derailleur. If you're having an issue downshifting (going to a bigger cog in the back), turn the barrel adjuster out, or counter-clockwise. This increases the tension on the cable if it has stretched or loosened a bit since it's initial adjustment. Go small increments, 1/2 turn or so at a time and check the shifting in between. Should be good to go.
EDIT: This all assumes that your derailleur was adjusted properly to begin with and is just shifting poorly now due to some cable stretch/seating. If that is not the case, then barrel adjusters are NOT the way to fix the shifting. They are just to fine tune it.