The aim of poker is to make the best possible five card hand from a combination of pocket (private) and community or board cards. There is a hierarchical hand structure that determines the winner.
However, it may be possible that two players finish with the same hand that has the same numerical value. For instance, two players both present a pair of Kings. In this scenario, the player hiding the next highest card, or kicker, would win.
Let's say the Player A has KJ and player B has K9. The board is Ace, King, Queen, Five and Four, of differing suites. As a flush can not be made, player A would win, with KK, Ace, Queen, Jack. Player B would have, KK, Ace, Queen and Nine.
So when playing a hand it is always important to consider your second card. If it is too low, it may be worth waiting for another chance to win.