Complex sentence: The tester is only aware of what the software is supposed to do, but not how i.e. when he enters a certain input, he gets a certain output; without being aware of how the output was produced in the first place.
Split into simpler sentences: The tester is only aware of what the software is supposed to do. When he enters a certain input, he gets a certain output, without being aware of how the output was produced in the first place.