Have you ever thought long and hard about a problem and then, suddenly, experienced an "ah-ha!" moment? If your answer is yes, then congratulations! You were enlightened or in possession of a clear understanding of what was otherwise mysterious.
The word enlightened comes from the Latin prefix en meaning "in, into" and the word lux meaning "light." Combine these meanings — "into the light" — and you're describing what it is that characterizes an enlightened person: a sense of clarity and understanding. People often seek to be enlightened spiritually, by taking pilgrimages, meditating, or looking to spiritual gurus like the Dalai Lama.