To save you looking it up, the 12 refers to the magnifying power of the binoculars (twelve fold). The 50 refers to the diameter (in millimetres) of the lens at the large end. The 341 ft @1,000 yards denotes the width of he field you would see. If you observed a brick wall which was 1,000 yards away, you would be able to see 341 feet of it from left to right.