In our everyday language, a fool is someone who lacks understanding of a lot of things. Because of this, he acts ridiculously to match his misunderstanding. But in the Bible, there was only one type of person who is always referred to as a fool.
In the Book of Psalms, a person is considered a fool if he does not recognize the existence of God. Why? Of all the countless evidences we can see everyday – nature, our own conscience, an intense design in the universe – all of those point to a Creator. Someone who does not see that is blind and considered a fool.
Another instance is in the Gospel of Luke wherein Jesus described a man who just got a harvest so big he destroyed his older barns and built a new one. In his sustenance, he said in his heart that he will just be eat, drink and be merry. The same thing, this person did not consider that God may take away his life at any moment, and so he was called a fool as well.