More context is needed as this expression might have different meanings according to context.
If it is said in a threatening context where the speaker was wronged by the other party and the speaker is talking about getting back at the other party, then I believe it means I will take my revenge on you.
If the speaker is talking about retrieving something that was taken from them, then I believe it means I will get back what's mine.
I hope this helps

This is a fairly colloquial English expression. You can also say "I'll get you back"

I would need to see the larger context to be 100% certain, but in colloquial English we would probably use the contractions "I'll" and "what's" to say this easily and quickly, giving the phrase a swift aggressive tone.