Okay, so, there was a shepherd boy who was looking after a flock of sheep. One day, he got bored and decided to pretend that a wolf was attacking the flock. The villagers came to help but there was no wolf. The next day, the shepherd cried wolf again, and the villagers came, and there was no wolf. This happened for days, with the villagers continuing to fall for the joke. One day, a wolf really did appear, but when the shepherd cried wolf, none of the villagers came because they thought he was lying again, and so all of the sheep were slaughtered.
It's basically a cautionary tale, saying if you tell a lie too much, when it actually happens, nobody will believe you.