That is obviously all about hand reading, and how you think, your opponent would react with different holdings. But let me try to create an example:
Preflop
You open JsTs from HJ and get called by BB in a cash game, where stacks are 100BB deep. Pot is now 6,5BB.
Flop
Flop comes Qd9d2s so you flopped an OESD. Opponent check, you bet 3,5BB as a C-bet, opponent call. Pot is now 13,5BB.
Turn
Turn is 4s so now you picked up a flushdraw as well. Opponent once again check, you fire again this time for 8BB, opponent once again call. Pot is now 29,5BB.
River
River is Jh improving your hand to second pair. Almost all draws missed. Only KT gutshot draw got there. Opponent once again check, and now the decision is on you. If you bet again, will your opponent call with any hands worse than yours? Probably not. If he still have TT or 9X in his range after facing 2 bets, he is likely letting it go now, at least if he is a decent player.
Will he fold any hands better than yours? Probably also not, because hands better than yours are mostly top pair or better, and the runout was not scary for a top pair. So a bet here will not win you more, when you have the best hand, and it will also not get your opponent to fold a better hand. Therefore you should check behind and take your showdown value, as its called.
Lets say instead that the river card was Ad. Now you are left with J high, and if you bet, you can almost certainly get some better hands to fold. Like K high or any hand, that has paired up. So you should at least consider to bet as a bluff. You dont always have to bluff, but you should consider it. Or lets say, that the river card was 8c. Now you made a straight, and your hand is simply the nuts. This is, what you were hoping for all the time, and now of course you have to bet and try to get some value.