Like others have said, it’s a window designed to help the store effectively manage the volume of customers throughout the day(s) with a new product launch. From my personal experience:
Going earlier than your pickup window - generally frowned upon but you might get lucky.
Going later than your pickup window - usually fine, but stores might prioritize customers who were specifically reserved that time slot over you. Usually by the end of the day they run out of walk-in inventory at my local stores and the lines for order pickups tend to be short so if you are going later go towards the end of the day.
When you preorder for pickup, your card is charged and you have technically paid for and purchased the phone. They will keep it for whatever number of days it says on the order, usually 7-14 days, so even if you can’t make it to the store the same day the phone is yours and they won’t give it away to someone else.