Row Level Security (RLS) is a powerful database feature, particularly in PostgreSQL, that allows you to define policies to restrict which rows a user can see or modify in a table. Instead of filtering data in your application layer, RLS enforces these access rules directly at the database level, providing a robust and centralized security mechanism.
For example, in a multi-tenant application, RLS can ensure that each user only sees the data belonging to their specific organization or account, even if they query the same table. You might define a policy that says: "A user can only select rows from the invoices table where the user_id column matches their authenticated user ID."
This is crucial for applications built with tools like Supabase, where client-side access (e.g., via the Supabase Anon Key) is common. RLS prevents unauthorized data exposure and simplifies your application code by offloading complex access control logic to the database. It's a fundamental building block for secure, scalable multi-user applications.