Irrational fears and phobias are the result of an overactive survival response in the brain. Your brain recognises something it has seen or heard (a trigger) and thinks this poses a threat to your safety.
The brain will then invoke symptoms of anxiety such as an increased heart rate, sweating, churning stomach, all designed to enable you to deal with this threat and to keep you safe.
With a phobia, that trigger can be innocent and non-threatening so the increase in anxiety is inappropriate and unhelpful. Hypnotherapy can disrupt this response so that the brain no longer sees the trigger as dangerous, thus getting rid of the phobia.