I once grew several tomato plants in the greenhouse. After a while hundreds of flowers appeared and I looked forward to harvesting ripe tomatoes. However, very few tomatoes appeared and the flowers withered and died. A little research taught me that tomatoes need to be pollinated and this usually happens by bees landing on the flowers, their buzzing wings causing enough vibration for the pollen to fall and pollinate the fruit.
Of course, you can’t always rely on bees to be buzzing around your greenhouse, but you can pollinate the tomatoes by using your finger to lightly tap the stalks, causing enough vibration to ensure pollination. I have read of some people recommending the use of an electric toothbrush to simulate the effect of bees, but, simply shaking the flower bearing stalks will produce the desired effect.