It could be fear of public speaking, fear of snakes, fear of closed in spaces, fear of spiders, fear of heights, fear of death…any sort of fear. Some people have multiple fears.
Fear is a strong emotion and is often used to build up to climactic moments and pivotal points in the plot. It is a good idea to brainstorm with a list of fears. And, if they are your fears, you may be more apt to make the scene more emotional. Sometimes it is nice to be able to put yourself in the shoes of your characters.