value bet, need some help here

[allyasia]

can somebody explain value bet and what's the situation to use it?
I found this to be another leak of my game.
not putting out that value bet for extra money, or put out that value bet, but it's dead money i'm putting in the middle.
if you have HH with explaination, that would be most appreciated.
for example why i think this is the leak that i need to patch.
IE 1) say 88, no raise PF, out of position.
flop 8K7, bet the flop, get call
Turn 2, bet the turn, get call.
River J, I ......value bet here? of course i would have hard time to call a raise here.

IE 2) AH 2H with on button
Flop 9H 3H 2C, villian bet pot, i call.
Turn JH, villian check, I bet the pot. villian call.
River 3C. villian check I value bet here? if i got action, my flush is certain no good.