The estimated delivery date is when your order is estimated to arrive at the address that you provided at checkout. Generally, the estimated delivery date is based on a combination of the processing time for the items you ordered (how long it takes the vendor to create and prepare them for shipment), the transit time (how long the package is typically in transit with the shipping carrier), and the current date.
For example: If the processing time for an item you purchased is 3-5 days, and the transit time for the shipping method you chose is usually 2-4 days, your estimated delivery date is 5-9 days from today.
If the vendor marks the order as shipped earlier than the expected ship date, then the estimated delivery date will be recalculated accordingly.