So what is it?
Suppose you create a fake of a class and do not configure the behavior of one of its method or property.
Strict mocking: If the test method invokes that method or property, then BAM !!! Say hello to Mr Exception.
Loose mocking: If the test method invokes that method or property, then you get the default value for that operation (which mostly will be a zero or a null).
FakeItEasy by default supports loose mocking. That said, it is not that FakeItEasy cannot support strict mocking. It does. More information here.