A value bet is a bet made to increase your profit, for example if you have the best hand in the final betting round and want your opponent to put more money in the pot. This bet has a positive expected value
and so increasing the pot size increases your equity and therefore, your winnings.
In other words can say ..A value bet is made when you're fairly confident you have your opponent beat and want to get one last bet out of him on the river.
Example: I had AA which made a set on the river and I bet out small as a value bet because I put my opponent on just two pair.