Flowers start as little buds on a plant. Then they grow larger and open up to reveal the flower. This process is called "blooming".

When a plant only has a few open flowers, you can say that it's "starting to bloom".

After most of the flowers are fully opened, you can describe it as "in bloom":

I see that your tulips are in bloom now.

And when the flowers start to turn brown, dry up, and go away, you say that they're "wilting".