A lieutenant is a commissioned officer in the armed forces or the police who can take command if her higher-ups aren't around.
Lieutenant is pronounced "loo-TEN-unt." If you're confused about all those vowels, blame the French, because the word comes from the French words lieu, meaning "place," and tenant meaning "holding." A lieutenant is someone who holds the place of authority for the person who really has it. If you start a super secret club, you should choose someone to be the lieutenant who can run meetings when you're not there.