It's simply to add an extension to a telephone number. Simply place a comma between the ten-digit phone number and extension number. When you tap on that contact to place a call, the comma will pause dialing between the phone and extension numbers, allowing the receiving end to accept the extension for what it is. So there's no need to enter the extension manually ever time you call a particular contact that requires one in your iPhone's address book when you get a comma involved. See iPhoneHacks for details.