Happy summed it up pretty well. A phobia is an irrational fear, so strong that it's very difficult to overcome, even though you know it's unreasonable. The good news for phobics is that they can largely overcome it by continually exposing themselves to their fear. It worked for me.
Fear is the realisation that something is dangerous or unpleasant. In that respect it's a safeguard but, when circumstances demand, we can do what's necessary in spite of the fear. A mother rushing into a burning house to save her child may be afraid but she'll still do what's necessary.
But here's a little known fact. The common barnyard chicken looks pathetic as it flutters around, sometimes lifting a foot or two from the ground as it attempts to fly, but most people aren't aware that chickens used to be able to fly as gracefully as swallows. Then a poultry farmer began to breed from a rooster with acrophobia and, since then, they've been afraid to fly.