An object mother is a kind of class used in testing to help create example objects that you use for testing.
When you write tests in a reasonably sized system, you find you have to create a lot of example data. If I want to test a sick pay calculation on an employee, I need an employee. But this isn't just a simple object - I'll need the employee's marital status, number of dependents, some employment and payroll history. Potentially this can be a lot of objects to create. This set data is generally referred to as the test fixture.
The first move is to create fixture in the setup method of an xunit test - that way it can be reused in multiple tests. But the trouble with this is often you need similar data in multiple test classes. At this point it makes sense to have a factory object that can return standard fixtures. Maybe 'John', an employee who just got hired last week; 'Heather' and employee who's been around for a decade.
Object Mother is just a catchy name for such a factory. The name was coined on a ThoughtWorks project at the turn of the century and it's catchy enough to have stuck.
The canned objects that the Object Mother produces become familiar to the team, often invading even discussions with the users. In this way they resemble the notion of personas - although they aren't always people. They could be insurance policies, supply contracts, whatever data a test framework needs. Using similar data on multiple tests helps people be familiar with the examples you're using.
These canned objects often aren't just right for a particular test, but often can be made right with some additional setup. "Let's take John and make him go off sick two months ago." Occasionally you'll need to add a new canned object to the mother, but try to tweak an existing one if you can - that way the reader of the test will understand quicker if they are familiar with the existing canned objects.
Object Mothers do have their faults. In particular there's a heavy coupling in that many tests will depend on the exact data in the mothers. As a result it's tricky should you want to change that standard data for any reason. Changes to classes will also result in the need to migrate the tests - although that will be an issue in any case.
Peter Schuh and Stephanie Punke wrote a paper on object mothers for XP Universe.