What do we mean when we say it today?
"The truth will come out" and "the truth will come to light" are both phrases that have their roots in this passage, and they're still used today. We hear people saying these kinds of things to one another all the time.
Just say your b-ball coach thinks you lied about how much you practiced at home. He might say, "Okay, the truth will come out." That's his way of saying, "Look, I don't actually believe you, but it will soon be clear whether you're lying or not. If you did practice, then you'll do well on the court. If you're lying and pretending to have dribbled way more at home than you did, guess what, you'll look silly during the game. So hey, the truth will find a way to reveal itself."
The meaning is still the same as when Lancelot first used it. Only not including the laugh-a-minute your-son-is-dead undertones.