Plain meaning
Start with the shortest useful explanation before going deeper.
A JavaScript runtime built on Chrome's V8 engine that enables server-side JavaScript execution. Node.js uses an event-driven, non-blocking I/O model. In the Solana ecosystem, Node.js is used for: running Anchor tests (Mocha/Jest), backend services, transaction bots, indexers, and CLI tools. npm/yarn/pnpm manage JavaScript package dependencies.