Hello @Louise154
When you made your inquiry, the host pre approved you. This gives you the option to book whithin 24 hours. You do not need to. It was just an offer by the host to allow you to book if you want to. The other thing that guests might do is "request to book" which means that they know they want to stay, and once the host approves it, the booking is done immediately.
When you receive an inquiry from a potential guest, the company wants you to approve (or decline) the inquiry within 24 hours, so often it will be approved without your asking. Again, only book if you want to.