This document discusses using PostgreSQL schemas to implement multi-tenancy in Ruby on Rails applications. It explains that schemas allow separating data for each tenant into a namespace or group of tables. This allows a single Rails app and database to serve multiple tenants (e.g. individual user blogs) in an isolated manner without needing separate app instances. The document provides examples of creating, inserting, and querying data in different schemas to demonstrate how Rails and PostgreSQL work with schemas.