Phosphorus is an element found in large quanitities on the V homeworld.
The V homeworld is plentiful in phosphorus because the V eggs have a protected shell made of phosphorus.
When Valerie was pregnant with a hybrid, she was beginning to feel tired. Dr. Lauren Pearlmen said that the pregnancy was draining Valerie of phosphorus, and that if she is not given a supplement, she will die. Ryan risked his own life by going on the mothership to get the phosphorus, and managed to get it without being captured but at the cost of Georgie's life.
In Red Rain, it is now known what the true purpose of the Red Sky, to disperse large quantities of phosphorus, the quantity found commonly on the V homeworld so the eggs have a protective casing on them. The Red Sky send out red rain which went onto the Earth below. The purpose was to change the Earth to a V environment, probably for the reason so the V's can live on Earth. The Red Sky's red rain was in the food, the water and in the soil. But another reason was that the red rain could rain on the humans, so they could survive the pregnancy when carrying a hybrid.
When Erica was pregnant with Tyler, she was told that her blood had high levels of phosphorus in her blood and so wondered that if she was experimented on by the Visitors when she was pregnant with Tyler. Erica confronts the doctor who cared for her during her pregnancy, and discovers that the doctor is a Visitor. From the files retrieved, she discovers that many women across the globe were experimented upon, like her, but most of the male babies died. Tyler is only one of 29 boys who have survived into adulthood.
Anna was hoping that Tyler's phosphorus levels would be boosted by Red Rain for him and Lisa to breed. However, his body has been resisting the change in phosphorus, leading to Anna looking for an alternative mate for Lisa, Raphael. It seems that for either sex of human to procreate with a Visitor, a high level of phosphorus is needed.