Row Level Security
Like locked drawers in a shared cabinet, each row in a database can be read or changed only by people who pass its rules.
Think of a filing cabinet with many drawers. Everyone can see the cabinet, but each drawer only opens for the person on its label. Row Level Security, often called RLS, lets the database apply that idea to data: every row has rules about who may read or change it.
It protects customer data without putting that logic in the app screen by screen. In Supabase, RLS is on by default, so nothing can be read or written until you add clear policies.
In practice you write allow rules that mention the signed-in user, such as "a user may see rows where user_id equals their id." The database checks the rule on every query, which means each request for data.