In poker, the strength of a hand (how likely it is to be the best according to the rules of the game being played) is often called its value; however, in the context of poker strategy the term is more often used to describe a betting tactic, a bet for value. This bet (or raise) is intended to increase the size of the pot, by inducing opponents to call. A bet for value is in contrast to a bluff or a protection bet (though some bets may have a combination of these motives).
For a bet for value to be correct, a player must have a positive expectation, that is, he will win more than one bet for every bet he puts in the pot. Note that pot odds do not matter in this situation, because the factor here is whether it is more profitable to raise or call, rather than to call or fold. Betting for value can apply to both made hand and drawing hand situations, although in the latter situation it is less often correct, as the drawing hand's chances of winning are generally lower. Many made hands will win the pot more than 50% of the time, therefore a value bet is usually correct, even heads up.
For example, in a game of Texas hold 'em, a player has 8♣ 6♠ with a flop of 9♥ 7♦ 2♣, The player has an open-ended straight draw and so has 8 outs (four 10s and four 5s). With 47 unknown cards, the player will make the straight approximately 1 time for every 5 times he doesn't, thus a bet is profitable if 6 or more of his opponents will call the bet (he will win once (+6 bets) and lose 5 times (-5 bets), out of every 6 hands like this, resulting in an expectation of +1 bet). If he thinks that less than 6 opponents will call the bet, he would lose money and must simply call.