A Neon Twin is a synchronized copy of an RDS production database in Neon, created and maintained using a GitHub Action that automatically runs a nightly pg_dump of the RDS database and restores it to Neon, ensuring developers can quickly start building, testing, and taking advantage of Neon's rapid development features without disrupting production or staging environments. To set up a Neon Twin, developers need to know their RDS connection string, AWS deployment region, a Neon account, a Neon database in the same AWS region as the RDS database, GitHub repository access to Actions and Secrets, and can leverage GitHub Actions to automate the nightly sync process, optimizing development workflows and reducing non-prod cost by 40%. The setup also requires understanding limitations such as job execution time limits and SSL certificate requirements.