What do we mean when we say it today?
This is one of those famous quotes that Lit lovers (like Shmoop) love to throw around because it sounds cool, and because we get brownie points for quoting Shakespeare. All this quote is saying, at the heart of it, is that there's a darkness we're copping up to. It might be something bad that we did, or a lie that we told, but it's no good. We're owning it by acknowledging that the darkness (evil) is ours.
But we'll admit that most of the time, this quote is just saying "hey I know Shakespeare" more than anything else in our world today.