Definition of value bet
A value bet is when a player bets with one hand and hopes to be called by a worse hand. A value bet is when you have a good hand to call. Sometimes a player can bet with a good hand and be called by a better hand. In this case, the player is said to have "value-own", i.e., he or she bet in the belief that he or she was going to get money from a worse hand, but in the end, simply lost more money with the worse hand of the two.
A value bet is the opposite of a bluff
A value bet is the opposite of a bluff. When you bluff, you have a weak hand and hope your opponent folds. When you value bet, you have a good hand and hope your opponent calls with worse. The size of the value bet should always be judged by the strength of the hand you suspect your opponent has rather than based on the strength of your own hand. Even with a very strong hand, if you think your opponent is weak, you should bet small to encourage your opponent to call with his worst hand, which would not be able to afford to call a big bet.