Use it to configure Mockito. For now there are not many configuration options but it may change in future.
In most cases you don't really need to configure Mockito. For example in case of working with legacy code, when you might want to have different 'mocking style' this interface might be helpful. A reason of configuring Mockito might be if you disagree with the {@link ReturnsEmptyValues} unstubbed mocks return.
To configure Mockito create exactly org.mockito.configuration.MockitoConfiguration class that implements this interface.
Configuring Mockito is completely optional - nothing happens if there isn't any org.mockito.configuration.MockitoConfiguration on the classpath.
org.mockito.configuration.MockitoConfiguration must implement IMockitoConfiguration or extend {@link DefaultMockitoConfiguration}
Mockito will store single instance of org.mockito.configuration.MockitoConfiguration per thread (using ThreadLocal). For sanity of your tests, don't make the implementation stateful.
If you have comments on Mockito configuration feature don't hesitate to write to mockito@googlegroups.com