Elie Wiesel says that we must see humans not as an abstraction because, having lived through the Holocaust, he learned what it means to treat people as something other than human. It lets others manipulate, exploit, injure, and even kill them. When you see someone as less human or real than yourself, it's easier to do things that take advantage of that person. In the case of The Immortal Life of Henrietta Lacks, Lacks herself is the person being exploited first by her doctors and later by the larger medical community.
Henrietta Lacks was a poor black woman who was treated for cervical cancer. Without asking her—and without telling her what they were doing—the doctors used her tissue to create a line of cells called HeLa cells. They're used in labs to this day. Until Rebecca Skloot wrote The Immortal Life of Henrietta Lacks and spoke to Lacks's family, they didn't understand the significance of their relative's cells that are still being used to aid scientific progress in labs all over the world.
The doctors didn't see Lacks as a person. Instead, they saw her as an abstraction. She was a means to an end rather than an individual with rights, family, and friends. Wiesel's quote is used to illustrate how dangerous and negative that mindset is. When people have that mindset, it can affect the people they're thinking of in a negative, dehumanizing way.