That means both CJS2 (via require(...)) and ESM (via import { ... } from ... or await import(...)) source will load this package from the same entry points when using Node. This has several benefits, the foremost being: less code shipped/smaller package size, avoiding dual package hazard entirely, distributables are not packed/bundled/uglified, a drastically less complex build process, and CJS consumers aren't shafted.

Each entry point (i.e. ENTRY) in package.json's exports[ENTRY] object includes one or more export conditions. These entries may or may not include: an exports[ENTRY].types condition pointing to a type declaration file for TypeScript and IDEs, a exports[ENTRY].module condition pointing to (usually ESM) source for Webpack/Rollup, a exports[ENTRY].node and/or exports[ENTRY].default condition pointing to (usually CJS2) source for Node.js require/import and for browsers and other environments, and other conditions not enumerated here. Check the package.json file to see which export conditions are supported.

Note that, regardless of the { "type": "..." } specified in package.json, any JavaScript files written in ESM syntax (including distributables) will always have the .mjs extension. Note also that package.json may include the sideEffects key, which is almost always false for optimal tree shaking where appropriate.