Your immune system knows the difference between you and the rest of the Universe, and tries to attack any of the rest of the Universe that comes inside your body. But sometimes, some parts of your immune system get a bit over-excited, and try to attack your own cells. As an essential part of building a healthy immune system, the natural process of Programmed Cell Death finds those over-active immune cells, and tells them to commit suicide. Every time a woman has a menstrual bleed, the cells that lined the uterus have gone through their natural and Programmed Cell Death. But in the language of the cells of the human body, apoptosis means the normal and essential process called Programmed Cell Death.
Apoptosis is necessary in normal growth, but it's also necessary in fighting off attacks.
Suppose that some of your cells get infected by a virus or bacteria. Those infected cells usually get told to die, taking the virus or bacteria with them, and so they protect the rest of the body. Think about the cancers. In a cancer, the cells have ignored the instructions to commit suicide, and have kept on growing. Here, the natural Programmed Cell Death did not happen.