An occupier is the person who legally lives in the house, apartment, or other dwelling in question. If the census taker comes to your home, she'll want to know if you are the occupier of the house, how many people live there with you, and so on.
Occupier is based on occupy, which itself is from the Latin occupare, meaning to take over or possess. An occupier is also an army that lives and works in a conquered foreign country. After World War II, the American, British, French, and Russian armies were all occupiers of the German city of Berlin. Each country controlled its own areas, but eventually, the American, British, and French section became West Berlin, and the Russian section became East Berlin.