It is commonly know as dry set. During the fertilisation period, the pollen dries out. When this happens only miniature fruits are formed and they will not grow any bigger. Usually the high temperature and the dry atmosphere to blame for such an incident. It can also be caused by infertile pollen. To prevent this to happen to your tomatoes, you should open the ventilation of the greenhouse regularly, especially in the middle of the day. If you don’t have an automated system installed on your greenhouse and you know that you will be out all day, it is better to leave the ventilation open in the morning. In very dry conditions you can create some mist around the tomato plants by spraying some water around them, don’t overdo this as fungal disease love dump too, or just poor some water on the floor of your greenhouse to make the air humid.