"Whenever you do a thing, act as if all the world were watching."

This quote is really about integrity. Jefferson is saying: live in a way you’d be comfortable with everyone seeing, not just what you show on the surface. Think about the moments no one else notices, how you talk about someone when they’re not in the room, the effort you put into a project when your boss isn’t checking, the way you treat a server on a bad day.
Acting “as if all the world were watching” doesn’t mean performing for approval; it means letting your private self match your public self. It’s a quiet little test you can use: if a camera were on right now, would I still be proud of what I’m doing or saying? If the answer is yes most of the time, you’re probably living pretty close to your values.