If you add this to the end of a verb, you're saying Don't (verb)!
1. Take the dictionary form of the verb:
하다 - To do (ha-da)
2. Drop the '다' and add 지마세요
하지마세요 - Don't do it! (ha-ji-ma-se-yo)
3. If you're being informal, drop the 세요^^
하지마 - Don't do it! (ha-ji-ma!)
You've heard it plenty of times before:
보지마! - Don't look!
하지마! - Don't do it!
가지마! - Don't go!
울지마! - Don't cry!