Lesson 39: Reflexes with 自分
In english, you don't say "Jim broke Jim's watch" when there's just one Jim. You say "Jim broke his watch", or sometimes "Jim broke his own watch". You don't say "Jim hurt Jim", or "Jim hurt him", you say "Jim hurt himself".
"His own" and "himself" have the property of being "reflexive". You already know how it works, "reflexive" is just the jargon used for it. When you need to do something reflexive in japanese, 自分 fulfills this behavior.
自分の体を食べた It eats its own flesh
自分で遊ぶ Plays by himself
お前、自分で… You, on your own...
自分 is also used in some dialects (and systems of slang) as a general personal pronoun, usually for the first person, but sometimes the second. It can be used like that in normal speech too, since it's a vague way to talk about someone.