Yes, if you hit "accept" to a guest's request for dates, you are committing to allowing them to stay in your property for those dates at the price indicated. You shouldn't hit "accept" if you are still negotiating price or if there is some confusion with the dates. The dates will remain available for Instant book while you are going through this process.
When a guest instant books, the acceptance is immediate. They have met your criteria for booking and are then committed to the dates they requested.
If a guest inquires, maybe they are asking you some questions or want to know if you'll do something for them, you'll get an option to "decline" or "pre-approve". Pre-approving is just that, that you, as the host, are willing to make this commitment. This requires the guest to finalize the reservation and then the booking is complete.
When the booking is complete - the calendar at Airbnb is automatically updated, they will keep track of your email exhanges with the guest and put the guest on your "reservations" list. Yes, the implication is that once both parties accept, the booking is complete and the guest will arrive on the date of check in.
Be aware that once a reservation is on the calendar, there are penalties for cancelling. https://www.airbnb.com/help/article/990/how-do-host-cancellation-penalties-work
