A lot of people know what the term "action" refers to in relation to their guitar.
If you don't, then it's a really simple concept that you experience every day that you play the guitar - the action is the height of the string off the fingerboard.
For many players, however, changing the action on their guitar is something that they know absolutely nothing about.
If you don't know how to adjust the action of your guitar properly, doing so can leave your guitar confused and unplayable.
If you know the basics behind how it's done, you can make adjustments and possibly improve the playability of your guitar.
I'm going to address the acoustic guitar and how you can make simple adjustments to its' action.
Before I tell you exactly how that's done, I'm going to need to explain to you a little about how the action is determined on your acoustic.
If you haven't taken the time to learn what the various parts of your guitar are, let me explain: you've got a body and a neck attached to that body.
At the top of the neck, farthest from the body, there's a head that holds tuning pegs.
Moving down the neck towards the body, you have a nut that holds the strings in place, the fingerboard holding the frets in place and finally, you reach the body with the resonating cavity & the bridge at the end, where the strings are held in place on the body.
The action of your acoustic is determined by the height of the nut, the bow of the neck and the height of the bridge's saddle, the little ivory or plastic piece that's on the bridge just before the strings take the turn into the body cavity.
If you've got a nice guitar, or a decent one at least, you'll find that the nut has already been cut to the proper height.
I don't advocate you taking the strings off and filing away at the nut, as your can leave the guitar unplayable if done wrong, but if the other adjustments listed here don't help you, it's one alternative you can try.
The neck has a truss rod through the middle, that's accessible at either end of the neck.
You may have to remove the plate on the head to access it, it's usually held on by a couple of screws and has the manufacturer's name on it.
Again, this is not something that you usually need to adjust.
The truss rod was put in place in case of any warping of the neck.
This way if the guitar is warped and the action is negatively effected, it can be corrected.
From factory, every guitar should have the neck set up properly and you shouldn't need to fiddle with the truss rod.
Finally, the saddle that's located in the bridge can also be adjusted.
Again, exercising a little caution is imperative, as you'll have to remove the saddle once the strings are off and file it down.
Some guitars do not allow you to remove the saddle, and if this is the case, this adjustment cannot be done.
If you can, however, remove your saddle, making small adjustments is the way to go.
You're going to be doing all these adjustments at your own risk, as each time you modify your guitar, you're changing dozens of different variable that will effect the tone of the guitar.
You'll want to take note of what's happened to your acoustic, if you once had good action but now you don't.
Look at the neck & see if it's warped.
Place your fingers on the first and twelfth frets and look at the neck from the side, as if you were holding it in your lap and playing.
If you see excessive bowing the middle, this clues you in to warping and you may need to adjust the truss rod.
Also take a look behind the bridge, towards the peg for the strap.
Often times there is visible bulging just below the bridge, which will cause the action of the guitar to change and go out of whack.
Both of these problems can be fixed, but they're most likely outside your area of expertise & should be brought to a professional.
If you cannot make very minute adjustments to any of the things listed above and bring your acoustic's action back into action, then it's time to bring it to your local music shop for it's yearly physical.
Again, the action of a guitar is a complex thing, and making too many adjustments can potentially ruin your guitars action.
If the acoustic you own doesn't have good action, and never has, consider getting a new one.
If you once had a good guitar, but now it's in shambles, consider taking it in if you can't fix the problem with a few easy adjustments.
Good luck!