What is a truss rod?
A guitar has steel strings (obvi) that create tension (obvi). Now this tension pulls the head of your guitar towards your bridge and creates a bow in the neck. If you hold your guitar out and look at the neck you might even be able to see it bowing. Now there is supposed to be a slight bow (very slight) but if your neck bows too far this can create a couple problems. To counteract these problems there is a steel rod inside your guitar neck that can be adjusted to reverse the bowing effect and keep the neck where it belongs. This is the truss rod.
If your guitar is bowing outwards (that is the head is moving up towards the body) what happens is that the middle section (think 7-12th frets) is farther away from the strings than your lower or higher frets. Meaning that you have to press harder than normal at those frets just to play the string properly (the higher the string the harder you have to press). If its really bad this can make the guitar extremely difficult to play. The difficulty in playing can lead to a little buzzing at these frets as well as intonation problems too. So if playing your guitar gets more difficult as you go higher up the neck towards the 12th fret you have this problem.
Now picture you have a problem with the guitar bowing the opposite direction. Now your middle section is too close to the strings. What happens here is that horrible buzzing or dead note sound no matter how cleanly you play the note or what you do. I find this is really common on bass strings when it starts to happen. So if you have a string consistently buzzing whenever you play it (especially at lower frets like the first five) you have this problem.
Now if you are a nerd you can also check for proper bowing with a feeler gauge and a capo and the common wisdom is do this twice a year (summer and winter). Personally I just wait until I have one of those problems and then adjust it.
How to adjust the truss rod.
Your going to need an allen wrench for starters second you need to know where it is. I won't cover every location the two most common are at the head of the guitar. If you see a little divet there or if you have a small plastic cover on the head (just above the very first fret) that is where the truss rod is (you just need to remove that cover). Or on Acoustics its typically just inside the sound hole if you look into the hole towards the head. Obviously you stick the allen wrench in the hole. Make sure to get a proper sized one you don't want to strip the screw.
Now this is important
do not turn the truss rod more than 1/4 turn at a time. It can/will break your guitar
If you have the first problem (where the string hieght increases and it gets harder to play as you move towards the 12th fret) you need to tighten (turn clockwise) the allen wrench. You might consider loosening the strings a little because it will tighten the strings. You are aiming for 1/8 to a 1/4 turn.
Likewise if you have the second problem (buzzing open or first five frets especially) then loosen (turn counter clockwise).
After that I would tune the guitar and then let it sit for 24 - 48 hrs. Then see if the problem went away. If the problem is there after 24-48 hrs turn it 1/8 - 1/4 turn again and wait another 24-48 hrs.
This stuff might not apply to you if:
You only play classical guitars. They don't have truss rods (don't need them).
If you are mortified about breaking stuff(its not hard to do but if you overdo it you can break stuff)/ don't know how to use an allen wrench.
3.If you have the cheapest of cheap guitars. (those supposedly don't have adjustable truss rods. But My first guitar was a FirstAct (a cheapo brand) and it had an adjustable one.)
That's all folks. Maybe we talk about action and intonation next?