Poker is a card game where players place bets on the strength of their hands. It has many variations but all have the same basic elements. A player places an ante wager and/or a pair plus wager and then is dealt cards by the dealer. They then have to decide whether to play their hand or not. Optimum strategy says to play any hand greater than Queen, Six and Four and to fold all other hands.
The dealer shuffles the cards and then deals them to each player one at a time. Each player then has to place a bet (the amount they placed as the ante). Players may then choose to raise that bet or not. Eventually, all the players have to either raise their bets or drop out of the game.
If a player has the best hand, they win and all of the players put their bets into the pot. Players place these bets voluntarily because they believe that the bet has positive expected value or because they want to bluff other players for various strategic reasons.
To be a good poker player you need to develop quick instincts. One way to do this is to watch other players and learn their tells (eye movements, idiosyncrasies, betting behavior etc). You also need to understand the rules of the game well. Another way to improve your poker skills is to practice a lot and read books on the subject.