"Mock" in this sense means "imitation," and that's exactly what a mock object does. Mock objects imitate the real objects that make up your program, without actually being those objects or relying on them in any way.
Instead of doing whatever the real object would do, a mock object performs predefined simple operations that look like what the real object should do. That means its methods return appropriate values (which you told it to return) or raise appropriate exceptions (which you told it to raise). A mock object is like a mockingbird; imitating the calls of other birds without comprehending them.
We've already used one mock object in our earlier work when we replaced
time.time with an object (in Python, functions ...