Most SaaS products are multi-tenant. Customers share the same application and infrastructure, which keeps costs down and makes updates simple, but each tenant's data is isolated. Isolation can be enforced at the application level, in the database with row-level security, or with separate databases.
Getting isolation right is fundamental. A bug that lets one customer see another's data is among the most serious failures a SaaS product can have, so tenancy should be enforced in more than one layer and covered by tests.