Enlightening is the -ing form of enlighten. As part of the verb, it means someone is explaining something so another person understands: she is enlightening us. Much more often, though, it is an adjective. An enlightening book, talk, or film is one that makes you understand something you did not understand before. It praises the thing: not only was it interesting, it actually taught you. Though actually, people sometimes say that was enlightening just to be polite after a long, dry talk — so listen to the voice, not only the word.

有啟發性的；讓人豁然開朗的。動詞進行式則是「正在開導／說明」。