"Closed sale" means that the loan, (if not an all cash transaction), has been funded and the property recorded. We're fortunate in Orange County because recording can take place on the same day as the funding. Elsewhere there is at least a one-day delay.
It is when a transaction between a buyer and seller becomes official. The Seller receives their money, the buyers the keys to the house and all the documentstion (mortgage, deed etc) is filed at the court house in the county where the property is located.
What the agent said below is true. Once all the conditions of a real estate transaction have been met, funds have transferred and the deed is recorded with the county is when a sale is closed. That could be in one day or more depending upon the area and the transaction.