Betting on the river?

[Big John D]

I hear people saying things like this all the time:

"Better hands aren't folding, and worse hands aren't calling."

I understand this, sort of. But I'm not real sure what kinds of things I need to consider in order to make this determination in the heat of the moment. Does it depend on how good my hand is? How good a "better" hand has to be? The kinds of players I'm up against? Where do I start thinking about this?

I know this question is pretty vague. But my understanding of betting on the river is also pretty vague. Can anyone shed any light on it?