721
Every NFT conforms to the ERC-721 standard. ERC-721 is a defined interface of functions and data structures that smart contracts must implement, or inherit from, to be considered a Non-Fungible Token. "721" fills the letters "NFT" with the actual Solidity function definitions and implementations specified in the ERC-721 standard.
burn
In a smart contract, the action of burning a token means transferring that token to an inaccessible Ethereum address. An inaccessible address is usually known as the zero-address. "Burn" is a visual representation of this process. The fire contains the Solidity code that implements the "_burn()" function in the ERC-721 standard, and the smoke cloud includes the characters "0x0" to represent the zero-address.
daofork
In block 1920000, a hard-fork occurred on the Ethereum Blockchain to help recover funds stolen in the DAO Hack. The fork consisted of a new contract deployed to the Ethereum Blockchain with a single "withdraw()" function allowing users to recover lost funds. "Fork" is a visual representation of this momentous hard-fork and consists of an interweaving fork filled with the actual Solidity implementation of the WithdrawDAO contract.
merkle
A critical data structure underlying the Ethereum Blockchain is the Merkle Patricia Trie, which allows for easy transaction verification and state transitions. "Merkle" is a visual representation of a Transaction Merkle Patricia Trie. The tree's roots contain real transaction hashes that lead up through the tree trunk to a root transaction hash. The transaction data is from a specific Ethereum block indicated in the bottom right corner of the GIF.