@Cherie71 A few pointers
The 24 hrs notice doesn't actually mean 24 hrs. So a person can't book at 12.04 am on the day of check in but they can book at 11.59pm the night before.
By checking the box allowing reservation requests it means hosts with Instant Book allow guests to request a stay right up to the point of check in (or beyond). We find this handy as we are often ready but don't want to be forced to be ready.
The only downside of checking this box is that you will upset Airbnb if you reject too many booking requests.