Other People's Opinions
In a society where interaction is part of everyday life, and other people form a vital part of life, other people's opinions are actually important in where we fit into society. However, we often think we know what other people are thinking and base our actions around those 'apparent' opinions.
Yet how do we actually know whether our assumptions about other people's opinions are true or not? Frankly, we don't. This is where Fundamental Attribution Error comes in. To put it into plain Engligh, it means that we make a basic error in assuming other people's thoughts without checking or otherwise knowing whether we are right or not in that assumption.
We base these assumptions on our schema of world (another Psychology term which means a system of beliefs we have about how world works from how to order food in a restaurant to how we 'should' live our lives). These schema are made up from our experiences in world, with some schema being made up from other people's schema which we have picked up along way: such as our parents' and friends' schemas.
Now mostly, these errors are fairly harmless: it is when we start living our lives entirely around what we think other people are thinking that it can become 'dangerous'. Many people fall into trap of becoming practically obsessive about what other people are thinking of how we behave that everything that we do becomes subject to what we think other people are thinking.