This sentence means that the plum flowers are white and bloom very luxuriantly, one next to the other without any gaps, so that the whole tree looks white.
It is said that when Li Bai was one year old, he grabbed a copy of "The Book of Songs". His father was very happy and thought that his son might become a famous poet when he grew up, so he wanted to get it for Li Bai. A good name to avoid future generations laughing at you for your lack of knowledge. Because he was cautious about naming his son, the more cautious he was, the less likely he was to come up with it. Until my son was seven years old, he still hadn't decided on a suitable name. In the spring of that year, Li Bai's father said to his wife and children: I want to write a spring quatrain, only two sentences, and you and your son will each add one to me, so we can make it work. One sentence is that the spring breeze brings warmth and flowers bloom, and the other sentence is that the golden spring comes first. Mother thought for a long time and said: The fire burns the apricot forest and the red clouds fall. Li Bai waited for his mother to finish speaking, then pointed to the plum tree in full bloom in the courtyard without thinking, and blurted out: The plum blossoms are in full bloom and a tree is white. When the father heard this, he clapped his hands and applauded. Sure enough, his son was talented in poetry. The more he thought about it, the more he liked it. As he thought about it, he suddenly thought that the first word of this poem was not his own surname. This last white character is so well used, it just says that a tree of plum blossoms is as holy as snow. So he named his son Li Bai.