You learned 空 means "sky," but it can also mean "empty." An empty car literally means "empty car," but in practice, you'll usually see this lit up on a taxi to mean open taxi. If you spot it, feel free to hail it down!

You might also see 空車 at a parking lot. In that case, it means there's an empty parking space available.

This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.