This is a companion package for the Rush tool. See the @microsoft/rush package for details.
The rush-lib package implements the rush.json config file loader and some other helpful utilities. Tools that want to reuse this functionality can install rush-lib alone to avoid inadvertently adding another "rush" executable to the command-line path (which might interfere with the globally installed Rush).
The @microsoft/rush version number is always exactly equal to the @microsoft/rush-lib version number that it depends on.
API documentation for this package: https://rushstack.io/pages/api/rush-lib/
Rush is part of the Rush Stack family of projects.