To win in poker, you need to have a better hand than your opponent, or at least convince them that your hand is better than theirs by successfully bluffing. Every hand in poker is ranked, starting with a pair and increasing all the way through to a royal flush.
Each hand has a rank, which is compared against the ranks of other hands participating in the showdown to decide who wins the pot. In high games, like Texas hold 'em and seven-card stud, the highest-ranking hands win. In low games, like razz, the lowest-ranking hands win.