Hello @Patricia796,
Instant book in currently turned on for your listing.
If you have not turned it on just recently that's why people could book without you having to approve them.
That's what instant book means. You know that instant book is on when you find your listing on the map and the big red button on its page says just "Book". If instant book is off it will say "Request to book".
If you wish to have guests send a request which you have to approve or decline then turn off instant book.
The article below has information on the instant book feature.
Answers-to-your-top-Instant-Book-questions
For instructions on how to turn it off click on "Help" and then type "instant book" in the "Ask a question ..." box.
Then click on the "How do I turn Instant Book on or off?" link.
Steve.