A plain-language explanation of near field communication and what happens when a phone taps a tag.
The short version
NFC stands for near field communication. It lets two devices exchange a small amount of data when they are held within a couple of centimetres of each other. It is the same family of technology behind contactless payments and transit cards.
How a tag works without a battery
An NFC tag is a tiny chip attached to an antenna. It has no battery. When a phone comes close, the phone's own radio field powers the chip just long enough for it to send back what it stores — in this case, a web address.
What the phone does with it
The phone reads the web address and offers to open it. On most modern phones this happens without opening an app first. The phone needs an internet connection to load the page, but the tag itself does not.