Tomatoes tend to crack when they receive irregular water. If your tomatoes have gone through a dry spell and you try to make up for it with frequent waterings, the inside of the tomato will plump up faster than the outside can stretch and grow. As a result the outer skin of the tomato splits open or cracks.
Tomatoes that crack are still edible. They just won't keep as long. You can prevent future tomatoes cracking by making sure that they receive water regularly, whether or not it rains. Mulching the area around the tomatoes will also help maintain ground moisture levels. Finally, look for varieties that are resistant to cracking.