Literally translated from Latin, it means "against the world". It has been used to describe maverick, 'lone-ranger' individuals who lack interest in culture and relative comparisons made by people that the individual deems 'ignorant' (which doesn't make them wrong, necessarily).
As mentioned in Brideshead Revisited (movie 2008), when Charles says he has no family and Sebastian replies "you have me, Sebastian and Charles, contra mundum".