Chickenpox (or chicken pox) is a disease. Usually it is children who get the disease, but adults can also get it. People who have it get blisters or spots, mostly on the body and in the face. Those blisters are filled with a liquid. At some point the blisters will drain, and the person will want to scratch them. Burst blisters usually become healthy without leaving scars, unless they become infected. The symptoms come in 2 or 3 waves, and usually include fever.
If a woman is going to have a baby, chickenpox can hurt both the mother and the baby very much.
Chickenpox is caused by a virus of the herpes family. People who had chickenpox in the past can also get a related disease called shingles later. Shingles is caused by the same virus but usually is in only one part of the body.