One day, he was bored and he thought it might be fun to mess around with the village people. He cried, with pretend fear in his voice, "Wolf! Help, there's a wolf!" All the village people began to race over with torches in their hands.
He laughed and laughed, and they grumbled and walked away.
He continued to do that same thing, day after day, and after the third day, they stopped believing him. The next day, the boy was taking care of the animals when a hungry wolf came and growled at him.
The boy, scared, cried "Wolf! Wolf!" But the people didn't believe him because he cried wolf too many times. The wolf ate all the sheep and cattle.
The wolf might be a metaphor for anything that turns up unexpected in life. Just to take one example, the wolf could be a symbol of an illness; if someone complains about imaginary aches and pains, the the doctor will be less likely to believe him when he actually IS in pain.
The meaning of this story is not to lie. When you lie, you lose the trust of others and it comes back to bite you in the end.