When someone's other half is better than them.
Back in the day this would have been used of marrying someone higher up in
society, but nowadays it's just generally when someone is punching above their weight in the
romance department. If you use it about yourself it's a compliment. If someone else uses it about you, they
basically think you're ugly.
"Did your wife make you those sandwiches? They look
amazing."
"I know! I married up."
OR
"
Buck-toothed
Jimmy just got married! Check out the wedding shots!"
"She is hot. That guy seriously married up!"