NDEF stands for: NFC Data Exchange Format. It is the standardized format, set by the NFC Forum, by which data is transferred between NFC tags and readers. When you tap your phone to an NFC tag, it reads the NDEF formatted message and is able to determine what action it needs to perform, and the details necessary to do so. These actions include opening a URL to a website, downloading contact information, connecting to a bluetooth device, and much more!
When you tap an NFC enabled smartphone to an NFC tag, the NDEF file will be the first thing that it looks for. Once the NDEF file is received, the phone will typically display a window asking if you would like to perform the specified action (opening a URL, downloading contact info, etc.). From here, it’s just a matter of providing your consent to execute the action.