A phobia is an intense fear of a situation or thing that is generally innocuous to others. The closer a person gets to the situation in terms of space and time the more anxious that person is likely to become.
The upshot of this is that we are likely to avoid the situation and this leads to further consolidation of our fear.
Examples of phobias may include blood, heights, spiders, snakes, needles etc. On a rational level you will often know that there is no real danger and may feel silly or embarrassed about your fear.
A phobia may often develop after a distressing and traumatic experience.