Flowers play the role of reproductive organs in plants. The pollen which contains the sperms fertilizes the ovules in a process called pollination.
To increase the chances for pollination, plants have evolved and modified themselves a lot. There is a large number of pollen produced in the flower. Each of them is sticky in nature and given features that makes it easy to stick on to insects.
Insects are the primary way in which flowers pollinate each other. To attract insects, flowers produce nectar. They also have other physical features like shape, color, odor, etc. that attracts insects to them. Once the insect sits on the flowers, hundreds of pollen cells stick on to its body. These are taken to other flowers and the fertilization of the flower completed.