Poker Position:
Some definitions, first: "Having position" on another player means he acts before you. "First position" means you act first. If you're dealt-in in fourth position, but the blinds and the player UTG ("Under The Gun", first to speak after the blinds) folded, you will be in first position for the rest of the hand (until you fold, at least). "Last position" is your position relative to all the other players still in the hand, meaning that even if you're not dealt in on the button, you're still in last position if no one will act after you (i.e. they players after you folded).

Position and Expected Value
I'll cut right to the chase: Position matters, because it will affect your expected value of any given hand. There are situations where you might gain a lot more from being in early position than in late position, but as a general rule of thumb, most hands get their best value in being last to act, and this is the key point: At showdown, all positions are the same - but being in last position means you'll have much better control of the pot size, and can therefore extract maximum value out of your monsters, while keeping it to a minimum when you expect you might be beat. The importance of this may be obvious, but let me state it for the record: As everyone, in the long run, gets dealt the same amount of good and bad cards, the difference between winners and losers is the amount they win when they win, and the amount they save when they lose.

Some examples:

You have
Kh 9h
and are in last position. The board is
7h Ac 8h 5d 6s

You're holding what is likely to be the best hand. You're pretty sure your opponent has at least a pair of aces, maybe two pair, maybe even a set. If you were to act before him, in this situation, and bet out on the river - would he raise? But now you don't have to worry about it - it's his problem. He has the tough decision of whether or not to bet his strong hand or not, knowing that if you have a 9, you'll raise it and he will have lost an extra bet. He also knows that if he checks, he risks missing an extra bet to a worse hand who would just check behind him.

You, having position on him, however, don't have this problem. You will bet if he checks, and raise if he bets. Your decision will always be the right one, his won't. As poker is about making the correct decisions, your position in this case gives you the advantage of information - or rather, denied your opponent the advantage of information. This is the most commonly stated advantage of position, but it's rarely stated exactly what you should do with it. Let me show you an example of when it might really matter:

You have
Ah As

and are last to act. The board shows
Ac Qc 8s 3h10c

There has been some action all throughout the hand, mostly between you and the first player to act, but there are two players who have simply been calling all the way to the river. Now, the first player bets, the second player raises, and the third player re-raises! What hands would suddenly come alive with that ten? Probably a straight and/or a flush. You had a great hand until the river, but you should probably lay it down. Your position saved you a lot of money here.

It's hard to trap when you're in position
Having position is a blessing and a curse. You're hard pressed to trap anyone when you're last to speak - if your monster is checked to you, you need to decide whether to bet to protect your hand, or if you should check to try and induce a bet from someone before you on the next street. If you bet your trip aces, chances are everyone will just fold, and you'll feel a bit bummed out over that, because you could have won a monster pot. And if you check, you're giving a free card that could give anyone a draw that will beat you. No fun.

Against typical players (whatever that means), I'll often represent strength in this spot. Slow-playing has its uses, but most people often fall victim to the FPS (Fancy Play Syndrome) where they're so caught up in their attempts to trap that they miss out on good ol' big pots. In fact, being in last position and betting into a board with two aces on it will so often be conceived as a steal attempt, that you're likely to get calls from people who just plain can't believe you actually have it. They won't raise you, but they'll call. Use that against them, and bet your hand when you have it. If nothing else, it might give you a better chance of stealing the pot those times where you bet with nothing out of last position, if their memory is good enough to recall you doing that with an actual hand from time to time.

In Closing
The reason hands go up in value when you're in last position is a combination of several elements, but the important lesson that I want you to learn here is that it's not because hands themselves get better, it's that your long-term expected profit from them increases because you will be able to make better decisions, and you'll get to control the table. The reason 10h-9h is playable from the button but not from UTG, for instance, is not only that you risk being raised pre-flop by some later player if you limp with it, it's also that the times when you actually hit your hand, you're not going to be able to extract enough money from it to make it worth the times you swing and miss.