When you receive an email that a new guest is ‘ready to book’, it means they have given us their payment details and have essentially pre-paid for the booking, should you accept it. The price you see in the offer reflects the price you've listed as either your hourly or daily rates. If the quoted price for the 'ready to book' is different than what you're accepting (e.g. the listed daily rate doesn't reflect your weekend rate, but they're requesting to book on a Saturday), then you can adjust the offer and send it back to them, without them getting charged.
It is essential that you either accept or respond (write a message) within 24-hours of receiving the request in order to prevent the request from expiring. Once it expires, you cannot accept the booking unless they request it again.