A more thorough explanation:

Definition: A lieutenant is a deputy or substitute for another person who acts with vicarious authority. The term is also used as a composite part of the title of many government and military officials who are subordinate to others. In the U.S. Army, a lieutenant is a commissioned officer next below captain, and in the U.S. Navy, an officer next below lieutenant commander.

• He sent his chief lieutenant to the meeting.
• The lieutenant governor is the second-highest-ranking official in many U.S. states.
• After completing Officer Candidate School, he was commissioned as a second lieutenant in the U.S. Army.
• The Navy lieutenant was responsible for the navigation of the ship.

The first example illustrates how a lieutenant can act as a deputy or substitute for another person. The second example shows how the term is used as part of a title for a government official who is subordinate to a higher-ranking official. The third and fourth examples demonstrate how the term is used in the U.S. military to denote a specific rank.