Learn German: Kommst du mit? — Na klar! - Are you coming along?

This sentence uses a separable verb, mitkommen (to come along). In a normal sentence or question, the main part of the verb (kommst) takes its usual spot, and the small prefix (mit) gets pushed to the very end of the sentence.

Does Kommst du mean "Do you come" or "Are you coming"?

It means both! German does not have an equivalent to the English "-ing" form. Depending on the context, Kommst du translates to "Do you come" or "Are you coming". Here, "Are you coming" fits perfectly.

What exactly does Na mean in the phrase Na klar?

Na is a casual filler word often used in spoken German, kind of like "Well" in English. While klar literally means "clear", putting them together as Na klar! is a very common, friendly way to say "Of course!" or "Sure thing!".

Can I just say Klar! without the Na?

Yes, absolutely. Answering with just Klar! is perfectly fine and also means "Sure!". Adding Na just makes your answer sound a bit more enthusiastic and native-like.