Authentication in Ruby on Rails 6 usually involves setting up user accounts with secure sign-up and log-in functionality. This process typically includes creating a User model, storing passwords securely with bcrypt, and building forms for registration and login. Rails provides helpers for managing sessions so users can stay logged in across requests. Popular gems like Devise can streamline the process by handling password encryption, validation, and authentication flows out of the box. Whether you roll your own or use a gem, proper authentication is key to protecting user data and providing a safe, reliable experience in your application.