What does No sabía que tú hablas español pero dónde lo has aprendido mean?

The sentence "No sabía que tú hablas español pero dónde lo has aprendido" translates to:

"I didn't know you spoke Spanish, but where did you learn it?"

Here's a breakdown of the sentence:

* No sabía: I didn't know

* que tú hablas español: that you speak Spanish

* pero: but

* dónde lo has aprendido: where did you learn it

The sentence expresses surprise that the person being addressed speaks Spanish, and then asks for more information about how they learned the language.