Menelaus is Helen's ex-husband and a commander in the Greek army. He's probably the most famous "cuckold" in literary history. ("Cuckold" is just an old-school term for a guy whose wife cheats on him. This word shows up a lot in this play.)
Because he's a "cuckold," he's the butt of several jokes about his status as the biggest chump of all time. Here's the most famous one:
Let Paris bleed, 'tis but a scar to scorn;
Paris gor'd with Menelaus' horn. (1.1.111-112)
Translation: When Troilus finds out that Menelaus has wounded Paris in battle, he says that Paris was probably "gor'd" with Menelaus' "horn." As we know, horns are a common symbol for cuckolded husbands (and, uh, penises). So, it's almost like Paris was sexually assaulting Menelaus by having sex with Helen—and then, by stabbing Paris, Menelaus assaults him right back. Because you know that a sword is just a pointy substitute for a penis, right?