Pollinate Flowers

If you are growing fruits (e.g. tomato), you should pollinate the flowers for fruit production.

Some fruits like tomatoes, peppers or eggplants are self-pollinators, you can pollinate the flower simply by hand. Touch the flower and let it shake and then the pollen will spread and pollenate other flowers.
You can also pollinate the flower with a soft brush (like makeup brush, paint brush or cotton bud), gently collect pollen from outside of flowers (male stamens) and transfer it to center (female pistil).