NFC (Near Field Communication) is a communication protocol which allows two devices to communicate and transfer data within a short distance. Some of the midrange to high-end Android devices has NFC support which allows you to utilize the feature.
NFC Tags are small chips which you can get in the form of stickers, cards, rings or even keychain. They can store a small amount of data in the range of hundreds of bytes. These are passive things which can work without a power supply or rely on other devices for power such as an NFC Tag depends on your Phone or the NFC readers to work.
Here I have bought a set of 5 tags for 4$ (₹250) from Amazon. The NFC tags I have can store data up to 144 bytes. There are different types of NFC tags which can store a larger amount of data.