Varicella is chickenpox, because chickenpox is caused by the varicella-zoster virus (VZV).
Chickenpox causes a blister-like rash, itching, tiredness, and fever. It's very contagious.
Being immune to Varicella means you're unlikely to get infected by this VZV. Usually the immune comes from vaccination. It's a two-dose vaccine, quite common in U.S.
