The truss rod is an engineering marvel. In its basic form it’s a long metal rod that runs through the length of the neck of your guitar, with one end fixed (most often near the high frets of the guitar) and one end accessible with a nut.
Tightening the nut will fix the rod and will make sure the wood can’t bend. Loosening the nut will allow the neck to flex a bit. Since wood is flexible and is subject to environmental changes, the truss rod is an essential piece of kit if you want to maintain a relatively constant level of action (the height of the strings relative to the frets). Hence, the truss rod plays a crucial part in the adjustment of the playability of your guitar.
Let’s explore the possibilities you have in setting up your guitar – with the truss rod as our focus – so grabbing that key and turning the nut will be less daunting.

The string height of your guitar is called the action. Some players prefer high action, like slide players. Others prefer their action to be super low. In any case, the string height has to be adjusted. The adjustments might be needed for a brand new guitar you just bought, but can also be done on a guitar you have owned for a considerable amount of time if you just want to revamp your guitar. Readjusting is also often recommended if you change string gauge. Thicker or thinner strings can require a different setup. And from time to time your guitar will need adjustment due to the effects of temperature fluctuation.
Let’s look at all the features of a guitar that influence the action in some way. The most obvious parts that have a role in the eventual action of your guitar are the nut and the bridge. If either is high, the action goes sky high and the guitar becomes unplayable. If they’re too low the strings will rattle against the frets. The nut should be high enough that the string at least clears the first fret (and for more on the importance of a proper nut profile, check out this article).

The bridge is an obvious way of raising or lowering the action. But raising or lowering the bridge alone can make the guitar less playable even if the strings are so low that they buzz against the frets. It strongly depends on where the buzz occurs. And that’s where the truss rod comes in!
If the string buzzes near the headstock but clears the first fret nicely, it’s not wise to raise the bridge because this would have the most effect on area of the string near the bridge itself. Loosening the truss rod a bit will relieve the neck, allowing the strings to pull the neck more forward, making the neck a bit more concave: in other words, raising the action a bit. If the string buzzes somewhere near the 12th fret or higher, loosening the truss rod will still flex the neck into a concave shape, and that shape might be the cause of the buzz. Raising the bridge or tightening the nut of the truss rod might do the trick of fixing the buzz.
The caveat is of course that the frets are already levelled. If the frets are not level the action will never be the way you want it all across the fretboard. Be careful though: over-tightening the truss rod nut might cause damage to the rod, and over loosening the nut might make it difficult to reattach the nut if it falls off. If you’re not confident with this stuff, take your guitar to a respected tech.