Stubs provide hard-coded answers to the calls performed during the test. Unlike fake objects, stubs do not have a working implementation. If the code calls a stubbed method getAllInvoices, the stub will return a hard-coded list of invoices.
Stubs are the most popular type of simulation. In most cases, all you need from a dependency is for it to return a value so the method under test can continue its execution. If we were testing a method that depends on this getAllInvoices method, we could stub it to return an empty list, then return a list with one element, then return a list with many elements, and so on. This would enable us to assert how the method under test would work for lists of various lengths being returned from the database.