Something that's abloom is flowering or blooming. When your rose bush is abloom, it's covered in huge yellow flowers.
If a plant is abloom, it's covered in flowers. Buds are opening, and blossoms are big and full. You can also use the adjective abloom figuratively, to mean "flourishing" or "healthy," as when your pink-cheeked friend looks abloom with good health from the fresh air. The prefix a- here means "in," and bloom has a Scandinavian root, the Old Norse blóm, "flower or blossom."