What is guitar action?
Guitar action can also be referred to as string action. Either way, it is an often guestimated measure of the height of the strings off of the fretboard.
Generally, the lower the string action the more playable a guitar tends to be. The higher the action, the more challenging to play.
Fairly cheap guitars have more frequent problems with higher string action, and guitars that are well-made (read: more expensive) have far more manageable string action.
Perhaps now you'll understand why guitarists will drop $3,000 on a guitar.
Why is it low guitar action critical for beginners?
Guitar action is measured in small increments, as low as one ten thousandth of an inch. Believe me when I say it, having a guitar with string action that is higher by one ten thousandths of an inch makes a difference.
There's a big difference between a guitar that has higher-than-average string action than a guitar with lower-than-average string action. If you are a beginner to guitar, you'll have to cover more territory on a guitar with higher-than-average string action:
With the following guitar, a much more expensive one, the string action is much lower. This means that it's far easier to play:
The less territory you have to cover, the less intense the feeling on your fingertips. This is especially true if you're brand new and you haven't developed callouses just yet.
String action is important. How do you find a decent guitar with low string action?
Armed with this information, it's time to put it to use as you are buying your first guitar.
You won't find out if the guitar you want has low string action if you don't hold it in your lap.* Take note of how high the strings are off of the neck. Even if you have no experience on guitar whatsoever, see how far you have to go to press the strings down. If you're really nerdy and want a precise measurement, then consider getting a gauge for guitar string height.
After a holding a number of guitars and comparing them, the question becomes this: How does it feel to play each one of these guitars? Not a bad question to ask, especially if you're committed to buying your first guitar.