For plants to reproduce, the pollen has to travel from the stamens to the stigma. This process is called pollination.
Animal pollination is carried out by birds and insects, such as bees. The colorful petals attract the animal to the flower. The animal comes to the flower to drink a sweet liquid called nectar. Pollen from the stamens sticks to the animal's body. When the animal travels to another plant, some of the pollen from its body is transferred to the other plant.
Wind pollination is used by plants like trees or grasses which don't have colorful petals to attract animals. The wind blows pollen from the stamens of these plants to the stigma.
Fertilization occurs when pollen joins with an ovule (the same as when in humans sperm joins an egg, remember?). This process happens in the stigma. The fertilized ovule becomes a seed. The stigma develops and forms a fruit around the seed. When a seed lands in soil it can germinate and a new plant can grow.