Rational fears are important to survival. Rational fears would be things like, concern that your young child may wander into the street. A rational solution would be to stay with that child and watch him and teach him the dangers of going into the street.
But what about those fears that prevent you from living a fulfilled life? Irrational fears do just that. They are an unreasonable reaction to a situation or event. The fear of being in a burning house is reasonable. The fear of being in a house because it might catch on fire is unreasonable.