A dimorphous flower is a flower that can look different depending on whether it is a boy or a girl flower. Imagine your friends have two different hairstyles depending on whether they are a boy or a girl. That's kind of what dimorphous flowers are like!
Sometimes the boy flower has bigger petals and a bigger middle part where the pollen is made. The girl flower might have smaller petals and a smaller middle part where a baby seed will grow if it gets fertilized by pollen from the boy flower. This way, the boy and girl flowers can work together to make sure they can make new baby plants!