A value bet aiming at a narrow edge.
Bet the river only when you are ahead and worse hands call more than 50% of the time. Example. Betting the river with TPGK (Top Pair Good Kicker) is thin value. It is right against players who don't fold, but against a tight player (nit), you are behind whenever called, so don't bet. Bet top pair + good kicker against up to 3 opponents, and second pair heads-up. It is the core idea of Limit Hold'em.