A pre-approval is a way for a host to let you know if their listing is available for the dates you’ve inquired about and that they’re inviting you to book. Once you're pre-approved, you can automatically book your reservation for those dates without waiting for another response from your host.
You have 24 hours to accept a pre-approval before it expires. If more than 24 hours have passed, you can still accept the pre-approval, but your host will need to approve your request before your booking is confirmed.