PEG.js is a simple but powerful parser generator for JavaScript. It enables you to define a grammar using Parsing Expression Grammar (PEG) notation and automatically generates a JavaScript parser. It's great for creating custom languages, data formats, or complex configuration files. PEG.js offers features like error reporting, backtracking control, and source code mapping, making it ideal for projects needing reliable and customizable parsing capabilities within web browsers or Node.js environments.