In Texas Holdem, position simply refers to where you are seated at the table in relation to the dealer. Position seems simple enough. However, your position at the table can greatly effect your advantage in the game, both positively and negatively.
The Small Blind is the position to the immediate left of the dealer, called as such because this player must put up the small blind bet at the beginning of the game. Not only is the Small Blind at a disadvantage because they have to put up a blind bet, but also because they are the first in the betting order, meaning that they have to decide to call, fold or raise before any other players.
To the left of the Small Blind is the Big Blind, the player who must put up the big blind bet at the beginning of the game. This player has the next-to-worst position after the Small Blind because they have the advantage of seeing how the Small Blind bets before they decide to call, fold or raise, but the majority of players in the game still get to bet after the Big Blind.
To the left of the Big Blind is the Under the Gun player, who is also not in a great position. However, the closer you get to the Dealer, who is the last to bet, the larger your advantage. The Dealer has the biggest advantage as he gets to bet after seeing what all of the other players have done.
Luckily, no player is stuck in any one position for long. At the beginning of each new game, the Dealer button is rotates one seat clockwise around the table so the Small Blind becomes the Dealer, the Big Blind becomes the Small Blind and so on.