Thin value

Betting a hand that only just beats the range that will call — second pair on a river, say. The margin is small, so the bet is correct and uncomfortable at the same time.

Because they add up. Most players check these hands and win nothing extra, so the pots they give away are exactly where a careful opponent profits. Over a year the difference is substantial.

By naming the calling range. If more of it is worse than your hand than better, bet — and keep the size small, since a large bet folds out the weak hands that made the bet worthwhile and is only called by the ones that beat you.