Dude, you sure ask some big questions! Definitely a hard one to answer.
If you get to a flop, there are two ways to win, making the best hand at sd and getting everyone else to fold before sd.
Good postflop play involves understanding the ranges of your opponents based on their actions and their reactions to your actions and in relation to the board. Some boards are better for your range based on the preflop and postflop actions, some boards are better for villain's range based on the same. Recognizing which boards are which, and which boards are unlikely to have helped anyone, and how to exploit those things are some of the keys to good postflop play.
Range examples - if you're the pf aggressor, A's and K's are generally more in your range than in villain's. Middle cards and middle/low monotone boards are generally more in villain's range. Flops with a low pair usually didn't help anyone.
Knowing when villain's actions means you're beat, and when you have very little chance to steal the pot, is pretty much knowing when to fold.
Knowing when it's ok to be passive and draw is part of this as well.
Entire books are available on this topic, gl!