When Bertram returns home from war, he sports a giant Band-Aid and a scar on the side of his face (4.5). Of course, everyone wants to know how he got it and why. Lafeu thinks it's a "scar nobly got" in battle and therefore sees it as a "liv'ry [uniform] of honor." In other words, Lafeu sees the scar as evidence that Bertram acted honorably while he was in Italy; this would suggest that maybe Bertram isn't such a bad guy after all. (He may have been a lousy husband to Helen, but, hey, he's a war hero, so maybe there's some hope for him yet.)
Not so quick. Lavatch then points out that, actually, the scar looks a lot like the kind a guy would get if a doctor had to treat him for syphilis (a venereal disease that causes skin eruptions). If this is the case, then the scar isn't so much a badge of honor as it is a sign of Bertram's shameful behavior. (Everyone knows that Bertram's been sleeping around and cheating on his wife every chance he can get.)
The thing is, we never find out how Bertram got the scar so we can't be sure if we're supposed to think of him as a war hero or a cheater. Our guess is that Shakespeare wants us to remember that Bertram is a combination of the two.