Well, you can make chess engines give you a numeric evaluation for several possible moves. These are typically tuned so that the value of a pawn is around 1 point. A grandmaster 1 or 2 points ahead can routinely convert to a won game, assuming he doesn't blunder.
So if the best move has an evaluation of +0.05 and the second best has -0.02 , the difference is probably a very subtle positional improvement (and the first move may not in fact be better; chess programs aren't perfect). If the best is +3.12 and the second is -0.02, and you can't see why, there's a concrete material tactic you're missing (or, less likely, a major, obvious positional devastation).
But, it can't tell you what you're missing, just the magnitude of the difference.
So if the best move has an evaluation of +0.05 and the second best has -0.02 , the difference is probably a very subtle positional improvement (and the first move may not in fact be better; chess programs aren't perfect). If the best is +3.12 and the second is -0.02, and you can't see why, there's a concrete material tactic you're missing (or, less likely, a major, obvious positional devastation).
But, it can't tell you what you're missing, just the magnitude of the difference.