web3.js is a collection of libraries which allow you to interact with a local or remote Ethereum node using HTTP, WebSocket or IPC connections. It is the most used library for interacting with Ethereum nodes, and is used by many Ethereum wallets and other applications.
Web3.js provides several APIs for interacting with the Ethereum blockchain, such as:
web3.eth: This module allows you to read and write data to the Ethereum blockchain, including reading and writing Ethereum transactions and interacting with smart contracts.
web3.shh: This module allows you to use the Whisper protocol, which is a messaging protocol that is built on top of Ethereum.
web3.net: This module allows you to get information about the Ethereum network, such as the current block number and the list of connected nodes.
Overall, web3.js is a powerful tool for interacting with Ethereum and building decentralized applications (dApps). If you're interested in building dApps or working with the Ethereum blockchain, you'll likely need to use web3.js.