A heavy smoker is someone who smokes at least 1 pack of cigarettes per day or more.
A light smoker is someone who generally smokes less than a pack of cigarettes per day or up to 10 cigarettes a day.
Being a smoker at all is not good for your health or anyone's health but being a heavy smoker is worse for your health and can shave years off your life and lead to a painful death early by lung cancer and making it hard to breathe.
My Grandpa used to smoke 2 packs of cigarettes per day and he died at age 60 of lung cancer and was in pain and coughed a lot during his life and especially during his last days on earth.