Some plants need a partner to reproduce. Pollen from one plant pollinates the stigma of another, and a seed is formed. But other plants can self-pollinate, a handy survival mechanism for a lonely plant.
The ability to self-pollinate turns up in cultivated tomatoes and canola, among other important crops, and sometimes it can be a nuisance for plant breeders and seed producers who want to develop highly desirable hybrid varieties and produce hybrid seed on a commercial scale. To get hybrid seed, they plant two different varieties in the same field to allow them to cross-pollinate.