If you're new to poker, one of the first terms you'll hear around is 'position'. More specifically, you'll usually hear one of two things, either a reference a player being in 'early', 'middle', or 'late' position, or in reference to one player 'having position' on another. Both of these terms will become much clearer with an explanation of the importance of position in poker.
In poker, position referers to where you sit relative to the other players. It's important because where you are sitting around the table determine the order is which you will act. Play moves to the left in poker, so the player sitting at your left will move after you. That means that that player will have more information when you did when he bets. This is known as having position on you, because of the where he is sitting, he act after you and there before always have more information then when he bets, a great advantage.
Early, middle, and late positions are references to where the players are sitting relative to the dealer. The first few players after the dealer are sitting in early position, they are the first to act and will have the least information when they make their decisions. Being in early position is unfortunant, but it does have one occasional advantage, bluffing from early position can be powerful because a bet from early position, when you have not seen any other players act, suggests a very strong hand. When sitting in early position, you only want to play the best of hands.
Middle position would be the next third of the players and are sort of a balance between early and late position. Middle position players have information about how the players in early position acted, and they bluffs can carry a little extra weight. Middle position players can play more hands because they have more information.
Late position is the best position to be in. It is the last few players to act, and they have the most information about how everyone else is playing. Because they have seen everyone bet, they have lots of information and can act accordingly. In addition, late position players will often steal many pots when everyone is holding a weak hand. If a late position players notices that no one before him has bet strictly, suggesting they have weak hands, he can often sweep in and steal the pot with a strong bet.
Because they have the most information, late position players also have the opportunity to play the most hands. As a general rule, being in late position is your best opportunity to play draws, hands that do not have all the cards they need yet, but will be very strong if the right card comes down. Because you get to act after everyone else, you can decide whether or not it is worth it to limp in with that flush draw.
That's the basics of understanding position play in poker. Learn it well and start to look for how position affects the way players bet and act in poker games. You will quickly discover just how important position is, and you will quickly take your game to the next level.