Row-level security, or RLS, is a rule enforced by the database itself that decides which rows a given user is allowed to read or change. Instead of trusting the application code to filter the data correctly every single time, the database guarantees it at the lowest level.
This matters enormously for multi-tenant SaaS, where many businesses share one application. With RLS in place, each tenant's queries can only ever return that tenant's own rows. It becomes technically impossible for one company to see another's data, even if a bug slips into the application above it.
Because RLS lives in the database and not in the surrounding code, it is one of the strongest tools there is for data isolation and privacy. When you are sizing up a SaaS vendor's security, asking how they isolate tenant data, and whether it is enforced down at the database layer, is a sharp question to lead with.