Hello @Jenny291,
For non instant book listings it means that there is another step in the reservation process after you click on "Request to book" before you'll be charged. That step is that the host must accept your booking. They have, I think, 24 hours to do that.
I've never booked an instant book listing so I don't know what the process is after you click on "Book".
There may be another step where you're asked if you wish to confirm the booking.
Either way, your credit card will be charged as soon as the reservation is accepted by the host.
See this article When am I charged for a reservation?
If the reservation is confirmed immediately when you click on "Book" your credit card will be charged then so saying "You won't be charged yet" would not be right in that case.
Steve.