My basic metabolic rate( BMR) is 2578. What does this mean in conjunction with calories?

Somehow I doubt your BMR is that high, but if it is, it means that this is the amount of calories you burn doing absolutely nothing. Your body has processes it needs to do throughout the day like breathing and digestion etc. These all burn calories and they make up the BMR. It means to maintain your weight (while not moving all day long), you need to eat 2578 calories.