We could say that a fixed idea is a frozen concept that has been divorced from its context and is being used generally without any further inspection. It is a thought that is taken as perpetual truth and never revised.
Imagine for a moment a person who knows with full certainty that anything yellow is poisonous to her. She will base her life on the avoidance of yellow things. You can appreciate the difficulty such a person would have in life. It would influence her eating habits, her shopping habits, her social life, her priorities in life, and so forth. If she sits down at a restaurant to order, part of her main criteria would be the color of the food. She would not get along well with people who like to wear yellow clothes. If she is offered gifts that are yellow she would regard it as a covert attempt of killing her. Likewise, if you attempt to persuade her that the color yellow is perfectly harmless. She already knows it isn't, and she isn't going to gamble with her life to investigate further. She will have ample experiences that back up her conviction about the color yellow. If you put her in a position of power then yellowness will be a factor in any decision that she makes. If she manages a grocery store then there will never seem to be any bananas in stock, and employees who use post-it notes don't seem to last very long.
This would be so far from the norm that most people would regard that person as rather insane. However, she might just be using her idea subtly behind the scenes, and it might never come out to an open discussion. You might then just notice that she has somewhat eccentric habits.
Fixed ideas is actually something the majority of the human race suffers from. There is a wide range of kinds of fixed ideas. Some of them will appear obviously insane, some of them sound quite reasonable. Some of them are continuously flaunted, others are so hidden that even the person herself doesn't know she is using them.
Fixed ideas are different from other types of material that we might address. They don't have any substance in themselves. They are ideas, thoughts. They aren't feelings, events, people, or things. They are kind of invisible. There is no tangible substance we can take and look at. They tend to be elusive for that reason. There might be substance to all the effects of the fixed idea: feelings, reactions, experiences, etc. But the idea in itself appears to be wholly intangible.
The power of a fixed idea lies exactly in the fact that it is an intangible idea without context. That means that it can be applied to a wide range of situations. It appears to fit all kinds of contexts. It becomes a general rule that is applicable everywhere.
Fixed ideas will tend to hinder the resolution of the subject matter they are applied to. For example, if our friend from above got into a big fight with somebody who offered her a glass of lemonade, it wouldn't quite resolve just from looking at what actually happened and what the emotional reactions were. No matter how much we scrutinize the traumatic incident in itself we wouldn't get an adequate resolution of it. Only if we address the fixed idea will we really get anywhere.
When fixed ideas are present in an area, perception is blocked to some extent. We can't count on that we can just examine the situation thoroughly, talk about it, and clear it up. The fixed idea will render that technique ineffective.
When fixed ideas are detected in a processing session they become the main target before anything else is addressed.