A Flush is a five-card hand in which every card is of the same suit--that is to say, all five cards are heart, all five are clubs, and so on. In most poker games, a flush is considered a strong hand: it beats most hands, including straights, and the hands that beat a flush--a full house, four of a kind, or a straight flush--are very hard to get. This isn't to say flushes are invincible, and in fact, flushes can be very difficult to play.
Very few poker games involve being dealt five cards all at once, so players can rarely start with a flush. This means that almost all flushes start out as draws: a player has two or three suited cards in early rounds of betting, and then makes their flush after more cards are dealt. This means you need to decide early on whether or not pursuing a flush draw is worth it--and you might still end up folding if players raise too aggressively for your flush draw to be worth it.
If two players have a flush, they can usually determine who has the stronger flush based on the highest suited card they have. This means if one player has made a flush using the Ace of diamonds, and the other has made their flush using the King of diamonds, the player with the Ace wins the pot. If both players used the Ace of diamonds because it is a community card or both players are using a wild card, then the second-highest card is used to break the tie--and then the third highest, fourth highest, and fifth highest. If both players have the same flush (say because the board in a game of Texas Hold'Em is all diamonds and no player has another diamond in their hand), then the players split the pot.
Even a flush can be beaten! A full house is a strong hand that beats a flush, so if your opponent is betting strong--and if the board has paired or they have a pair showing--then you should be aware that they might have you beat.
Also, be cautious not to overbet a baby flush--that is, a flush made with low suited cards. If you have the Four and Five of clubs in the hole, and the board comes up all clubs, don't assume you're the only one with a flush: one of your opponents might have a better flush of their own.