A lieutenant governor is like a deputy or assistant to the governor of a state or province. They are usually elected just like the governor and have similar responsibilities.
Their main job is to step in and take over if something happens to the governor. This could be if the governor gets sick or goes on vacation, or if something more serious happens like the governor resigning or passing away. If that happens, the lieutenant governor becomes the new governor and takes over all of their responsibilities.
The lieutenant governor might also have other important jobs like leading important committees, helping to make laws, or representing the state or province at important events.
Basically, the lieutenant governor is like the backup plan for the governor - someone who is ready to step up and take charge if needed.