Learn Finnish: Käveletkö? - Are you walking?

This is because of vowel harmony! The word kävelet contains the front vowel ä (along with the neutral vowel e). Because the word has a front vowel, any ending attached to it must also use a front vowel. Therefore, we add -kö instead of the back-vowel version -ko.

Where is the word for "you" in this sentence?

In Finnish, the ending of the verb tells us who is doing the action. The -t in kävelet means "you" (singular). Because this ending makes the subject obvious, the pronoun sinä (you) is often dropped.

Does käveletkö mean "Are you walking?" or "Do you walk?"

It can mean both! Finnish doesn't have a separate tense for ongoing actions like English does (the "-ing" form). Depending on the context, Käveletkö? translates to "Are you walking (right now)?" or "Do you walk (regularly)?"

How would I answer this question with "yes" or "no"?

In Finnish, you usually answer a yes/no question by repeating the verb. To say yes, you conjugate the verb in the "I" form: Kävelen (I am walking). To say no, you answer with the negative verb: En (I am not).