"You, but I don't know French well."
Here's the breakdown:
* Toi: You
* mais: but
* je ne sais: I don't know
* bien: well
* le français: French
A more grammatically correct way to say this would be:
* "Tu parles français, mais moi, je ne le parle pas bien." (You speak French, but I don't speak it well.)
This phrase is more natural and fluent in French.