Plants, just like animals, can be sexual or asexual organisms. Sexual organisms utilize two organisms to undergo fertilization in the reproduction of offspring. Asexual organisms reproduce either through division (mitosis, budding, fragmentation, etc), or via self-fertilization.
Just as there are male and female animals and animal anatomical parts, there are also male and female plants as well as male/ female parts of a flower. Pollen is considered part the male reproductive cell and is created by the anther that resides on top of the filament. Pollination is when the pollen lands on the female anatomy of the plant (stigma, style, and down to the ovary).
Some flowers contain both of these male and female parts. Other flowers house just the male part, or just the female part. If a flower houses both the male and female reproductive organs, then it will be able self-pollinate (asexual reproduction). If the parts are located on two separate plants, then the pollen from one plant must land on the stigma of a second. We call this cross-pollination (sexual reproduction).