The A column is the structure of the car between the windshield and the driver and passenger windows. The B Columns are subsequently the structure between the first and second row of seats (I think).
Notably the thick and angled A columns of modern cars create fairly large blind spots to the front left and right, while wide B columns are problematic for larger drivers, since the B column blocks visibility over the shoulder.
Well you need to know that they refer to columns in the car's body in the first place.