Bitcoin NFTs refer to NFTs (non-fungible tokens) minted on the Bitcoin network.
The process of issuing Bitcoin's NFT assets is actually the process of attaching arbitrary data (text/images) to the smallest unit of Bitcoin, "Satoshi". This process is also known as "engraving/inscription".
In December 2022, Bitcoin developer Casey Rodarmor released an open source software called ORD that runs on Bitcoin Core software.
ORD allows users to mint Bitcoin NFTs in two steps: 1) Enter arbitrary information on the Bitcoin blockchain, such as a string of text or images ("inscription", some translations call it inscriptions) ,2) Connect the uploaded inscription to a specific satoshi ("ordinal"). Satoshi is the smallest unit of Bitcoin. 1 BTC = 100,000,000 satoshi. The final product is an inscribed satoshi, otherwise known as a Bitcoin NFT.