"Keep it up" means continue doing what you are doing, and it usually refers to something positive. An example is "You are doing great work. Keep it up." This would mean that someone is complimenting your work and wants you to continue to do a good job.

Hi Vy, This is a phrase to compliment and encourage someone to maintain the good work he or she has done so far. Example: You have improved tremendously over the last three months. Keep it up! We can also write: keep up (something) to encourage someone to continue improving Example: Keep up the good work! I am so proud of you. Hope this helps. Cheers, Lance

The other answers are absolutely correct. This isn't directly related to your question but on the other side of things to 'keep something up' can also be used negatively. Examples 1. He never studies and stays out late every night. If he keeps this up he'll fail the semester.' 2. My has started going to the gym at 5 every morning. There's no way he'll keep that up.