Part of the reason that position matters is that the betting order can help to determine who stays in a hand and who folds a hand. Also, the type of betting will also play a factor in the betting action.
In a “Limit” game, the amount of bets are in increments of the big blind and usually no more then four raises of that amount can be made during any particular round of betting.
In a “Pot” limit game, the maximum bet amount is limited to the amount of the “Pot” that would result if everyone made a bet equal to the size of the big blind. As there can be up to four rounds of raises at each stage, the maximum amount become real large real fast.
Finally, “No” limit bets must be at least the amount of the big blind and can be as much as a player has in front of him. (He cannot just reach in his pockets and buy more chips during the play of a hand), hence the name, “No Limit”.
Using just “Limit” betting to explain position, I will describe the effects of implications of position. The player that is “under the gun”, or to the left of the big blind should have a good hand if the player wishes to call or raise the big blind. As there are nine other people after him that can react to his action, you can see what could happen. Up to three other players could also raise after the “under the gun” player. So it turns out that the “under the gun” player must or should have a very good starting hand if he decides to call or raise in early position. The usual range of starting hands for the “under the gun” position is AA, KK, QQ or AK suited. The other players that follow can decide to call or re-raise and can usually assume that the previous raiser has perhaps one of the strong starting hands that i mentioned (unless a bluff is being made).
In the pre-flop betting, the last player to act is the big blind. For the rest of the hand, the “dealer” or “button” then becomes the last one to act. The button is a vary favorable position to be in because that acts last in the rest of the rounds of betting. The “button” player gets to see what everyone else has done before he makes a decision. The later the position is, the less the starting requirements for hand becomes. This is because the latter player can decide if his hand is worth staying in the game with. With what is called a drawing hand, a hand that needs one or more cards from the common cards (the flop, the turn or forth street, the river or fifth street), the later player should limit the amount of bets called to a minimum to avoid large losses if the event that the later player does not make his hand.
Many poker books will suggest a range of hands for the early players, the middle players and the late player. These range of hands are just a guideline. As you play at a table and become faimliar with the way the other people play thier hands, you can adjust the type of opening hands to fit the current conditions. In all cases, you should never be predictable as that will give your opponents an edge as they will know what type of cards you might enter a hand with and will adjust thier play accordingly.