"Antebellum" - Latin for 'before' = "ante", Latin for 'war' = 'bellum" thus the phrase means 'before the war' or 'pre-war', in this case, specifically, the American Civil War.
Mean can mean you are being rude or in math terms it is adding a set of numbers then dividing by the total amount. ex. If your numbers are 1,2,and 3 add them to get 6 and divide by 3. You get 2.