A nom de guerre is a fake name (or pseudonym) that you use for a particular task or job. If you're a member of a roller derby team, you might adopt the nom de guerre of "Dora the Destroyer."
Originally, a nom de guerre was literally its French translation: "name of war." Starting around the fifteenth century, French army recruits were assigned official noms de guerre, usually relating to their hometowns or some distinctive physical characteristic, which acted like identification numbers. Today, most often a nom de guerre is simply a false name that you use in one particular situation or role.