Definition of thin value bet
A thin value bet is a value bet that you make on the river with a good hand, but which does not necessarily deserve to be a value bet on the river. This can be done when you have a bluffing image and don't have much credit on your bets or when your opponent is the type to call too often (calling station). A thin value bet is also when a hand is bet in value on the river after being bet in value on the flop and turn while being weak. A hand such as medium pair is not usually a hand that you would want to bet value on for 3 streets.
Thin value bets, especially one-on-one
Generally, this type of betting is done heads-up. A hand such as middle pair will rarely be bet in value when playing full ring, but when playing heads-up, this bet could be valid since at the showdown, the hands that win the pots will be weaker. Bluffing and aggression are also more present.