As an adjective, through means finished or done. As an adverb it can mean backward and forward, completely, up to and including, or all the way to the end. The preposition through means in and out of.
Through has lots of meanings. If you finished your homework you might shout — gleefully — "I'm through!" Those people who wouldn't be quiet talked through the entire movie. The first half of the alphabet are the letters A through M. If you've memorized the multiplication table, you know the whole thing through. And, of course, you walk through a door.