Complex sentence: In the fictional "Harry Potter" series, NEWTs is an acronym for Nastily Exhausting Wizarding Tests, a highly-advanced test of magical ability in a particular field that the Ministry of Magic administers to students before they graduate from Hogwarts.
Split into simpler sentences: In the fictional "Harry Potter" series, the Nastily Exhausting Wizarding Test, or NEWT, is a highly-advanced test of magical ability in a particular field. The Ministry of Magic administers the series of tests to students before they graduate from Hogwarts.