"Is this seat available?" "Are you available, or are you married?" As you can tell from these two examples, available means "not busy or not taken."
Available can be used to describe anything that's ready to be used like a restaurant table or a theater seat. It can also be used to describe a person who is not involved in a romantic relationship: "Available singles use online dating websites." When you use a resource, like the school library, you can say that you "avail yourself to the research opportunities made available to students." No matter how you slice it, available means free for the taking.