In poker, a thin value bet is a potential bet you make on the river with a marginal hand in order to extract the maximum amount of value.
It is about pushing edges and making correct positive expected value plays (+EV). Therefore you bet your poker chips when you think it is more likely than not you have the best hand, even though better hands will call as well.
Players can, over time, lose more money by choosing not to bet the river – and leaving value on the table – than by actually losing pots. So always look out for situations where you can value bet thin, even though it can often put you into more difficult situations such as being bluff raised. We make more money by making more uncomfortable decisions in poker and maximising our expected value.