In between the two polar opposites of hands in poker, those of the very strongest hands and the very weakest hands, exist a large number of marginal hands. The best players make most of their money through how they play the marginal hands (after all, anyone can fold five high or bet the nuts, but working out what to do with middle pair is much harder), and one of the most crucial concepts involved with marginal hands is showdown value.
One of the most common mistakes you see people make (even up to the very highest stakes) is misunderstanding showdown value.
A hand like five high has no show down value - it will not beat anything, and the only way it can win the pot is with a bluff. Conversely, the nut flush has complete showdown value - you know that no other hand can beat you and can act in any way you choose in pursuit of building the pot. A hand like middle pair is much more interesting though. It has some showdown value, in that it will beat a bluff or other marginal hand, but is unlikely to stand up against a strong hand.
Hands with some showdown value are nearly always hands to call with. They have little value as raising hands, as you do not want to play huge pots with them; and they have little value as bluffing hands, as your very weak hands (those with zero showdown value) are the hands to be bluffing with, as they have no other way to win. Hands with some showdown value are great hands to try and pick off bluffs with, as showing that you are capable of calling with them makes your opponent's life much harder, as they have to re adjust their bluffing frequencies in the face of marginal calls from you.
One of the most common mistakes you see people make (even up to the very highest stakes) is misunderstanding showdown value. They bluff with hands that have a decent chance of winning, or give up with their hands that have zero chance of winning. This is a fundamental mistake and one which will cost you dearly in the long run.
Becoming familiar with showdown value and the importance it has on your game is crucial to taking your game to the next level. Being able to think about the relative strength of your hand, showdown value, and the implications of it is critical to long term success.