Supplant is a variable substitution and interpolation library that processes strings containing expressions enclosed in `{{ }}` braces, replacing them with corresponding data values. It's designed as an agnostic, standalone template engine suitable for both client and server-side use, and is particularly useful for developers building dynamic templating systems or working with frameworks like brick and datastore. The library supports advanced features including expressions, filters, and property extraction, making it ideal for JavaScript developers needing flexible string interpolation capabilities.