Poker is a card game in which the goal is to form the best possible hand based on the cards you have, and win the pot at the end of each betting round. The pot is the sum total of all bets made by players at a table. When it is your turn to bet, you can either call (put in the same amount as the player to your left) or raise (put in more than the previous high bet). If you do not want to stay in your hand, you can fold (slide your cards into the dealer face-down).
After all players have two hole cards, a round of betting begins, initiated by mandatory bets called blinds placed into the pot by 2 players to the left of the dealer. During this round, players can also draw replacement cards to their hands. Once the betting is complete, one more card is dealt to the table, this is called the flop. Then there is another betting round, and then a showdown where the players reveal their cards to determine who has the best hand.
As a beginner, you should start out by playing at the lowest stakes available to you. This will help you avoid losing a large amount of money and learn the game without giving away your bankroll to stronger players. Remember that even the most successful professional poker players were once beginners, and it takes time to get a grasp on this complex game.