This is where ownership is legally transferred to the buyer. As the seller you will need to be prepared to give over any necessary documentation regarding the property and, depending on the arrangements made during negotiations, you may be required to have done something specific in order to close. Be sure and read all the documents. Mistakes are more common than you might think. 
I can save you time and money by being present during the closing to help explain the process and forms to you and make sure everything goes as planned. By being present during the closing, I mediate any last minute issues that may arise. Something always seems to come up!