[raze]

You have 88 in late position against one opponent.

Flop 7-5-3. He checks, you bet the pot, he calls.

Turn 7-5-3-A. He checks, you bet 2/3 pot, he calls.

River 7-5-3-A-6. He bets out small.

You can either fold, call the river (you were betting your hand for value on earlier streets, but now you might be happy to take a cheap showdown), or RAISE the river as a bluff, because clearly on this board you do not expect to get paid by a worse hand. Thus, you've taken a hand that you were betting for value, and with a river raise, turned it into a bluff.