NFC stands for Near Field Commmunication and is an international transmission standard for the wireless exchange of data over short distances (10-20cm). For data transmission via NFC, the smartphone is held on another NFC-enabled device (e.g. smart lock or wall reader) that activates certain applications. In the case of Tapkey: the signal to open the door.
Meanwhile, many smartphones are equipped with an NFC chip, others can be subsequently retrofitted with an NFC sticker (e.g. iPhone). The NFC sticker is then placed on the back of the device, replacing the built-in NFC chip.