To compliment someone or encourage them to keep doing what they’re doing. Examples: You’re doing a good job, keep it up! If you keep it up, you will succeed. You got a good grade in math. Keep it up! You can also use it to warn someone: If you keep it up, you’re going to get in trouble. In the negative sense it still means “to keep doing what you’re doing.”

You could use "keep it up" whenever someone is working on a goal, project, etc and you want to encourage them. Guess what Amany, Im loosing fat by working out at the gym daily. (you) Great, I hope you "keep it up". Here in America we also say "Keep up the work" or "keep up the great work" I hope this helps, Jerry (Teacher/turor)

Examples: Me: “I am body-building every day for the last six months to be as strong as Schwarzenegger is” My friend: “That’s great! Keep it up and you’ll reach your goal!” Me: “The exam is only two days ahead of me and I don’t think I’ll learn everything by then” My mum: “You need to keep it up (your daily learning routine) and you are going to pass that exam”