Essentially a nightmare is a powerful message. There is no simple answer, but if you succeed in working with a dream so as to understand why you are having it, then you won't need to have it again and it will go away. Sometimes people need professional help working with frightening dreams. Such dreams always represent some powerful psychological material that needs to be considered. Symbols seem to be the language of the unconscious, perhaps because dreaming is a non-linear kind of process, an intuitive and unconscious process.
Symbols can be a great source of amusement.
For example, someone ironing in the middle of a dream can represent 'irony' about a situation. Think of symbols as if they were a code you have to crack by thinking about what that image means to you. What someone else has to say about a dream symbol usually will have little meaning for you. There are some universal symbols, but these will also have a personal context and meaning that may be different for each individual. Most dream symbol lists I have seen are completely inaccurate.