At many games, your position is chosen for you, either at random or based on whichever seat is available. But if you have the option, consider your seat wisely: position is power. As such, you'll want to sit so that you have an advantage over stronger players to your right. You might think it's better to have weaker players at your right, so you can take advantage of them, but overall, it's always better to have the edge of your strongest opponents, so try to sit to their left.
Your position at the table varies based on your relationship to who deals the cards. In games with blinds, the first person after the big blind is called "under the gun," because they're first to act after all the cards are dealt. Some people abbreviate this position as "UTG." The position after that is abbreviated as "UTG+1," and the next is "UTG+2" and so on. If the table has lots of players, people after UTG+1 or UTG+2 might be called "MP," for middle position. There is then the "hijack," the third last to act; the cut-off or "CO," the second-last to act; and then the dealer, also called "on the button," which is the most powerful position: the last person to act, and the person with the most information before they get to make their move.
After the dealer, the players in the blinds play: small blind ("SB") and big blind ("BB").
The hijack seat is the third-to-last player to play, the position two seats to the left of the dealer. This player is in a position where they have a lot of information, and can make a big move before the cut-off or the dealer decide to do so. With enough information and perhaps a strong hand, the player in this position can "hijack" the button by scaring the other two behind and taking over the button in every subsequent round of betting.
Poker tables can vary a lot, but most have a max of 8-10 players. But some games might structures with fewer players as a max, with some tournaments doing heads-up tables: every player plays just one player at a time.
Lower maxes are especially important in games where players get a lot of cards, like seven-card stud: the deck just doesn't have enough cards. But even if the maximum number is low, there is a chance the dealer will have to deal cards face up so that every player can use a community card to make their hands.
Without a doubt, the best position in poker is the dealer: the player who is last to act and has all the information before they have to make a decision on their plan of attack. This gives them the edge of knowing how every player has acted before they have to do so themselves.
In some stud games, the cards determine who acts last: the person with the strongest showing hand acts first, so the player to their right is the last to act. This means who is last to act can change as cards are dealt and players improve their showing hands. You'll need to keep track and alter your course of action accordingly!
The cutoff, or CO, is the last player to act before the dealer. This player is in a tricky spot: they have a lot of information, but not all of it. This player needs to consider the fact that they can command a lot of power if they can convince the dealer to fold. But if they make a big bet, and the dealer responds strongly, now the CO is in the position of having represented strength while the player to their left is acting even stronger. The CO needs to be careful to exploit their strong position without getting one upped.
Position determines how many players act before you, which means it determines how much information you have--and in poker, information is power. If you can see everyone else's actions first, you can change your action according to how you read them: do they seem strong, weak, or unsure? You can respond to their actions, and potentially win the hand outright by making a big move--or convince them to put more chips in the pot by stringing them along.
Conversely, if players are behind you, you need to consider what they might do: are they aggressive, or are they passive, and how should you act without knowing how they'll respond? You need to predict their actions, which is harder to do in earlier positions--plus, they might decide that their position gives them an edge and they can try to intimidate you accordingly. This is why position is so important: it affects every players' action, and you need to be aware of it.