A civilian is a member of the population who is not a crew member. They are generated as part of contracts, and are usually temporary occupants of your ship.
Civilians have the same life requirements as crew members, and are unable to wear spacesuits. Civilians will die in the habitation deck should oxygen levels in the main deck drop too low, CO2 or sewage levels get too high, or if there is not enough food and water in storage for a long span of time.
Upon boarding your ship, civilians will enter the habitation deck through the Ladder where they will remain until the contract that generated them is complete, at which time they will disembark your ship via a cargo shuttle.