- Some symbols just don't mean anything. This is called grounded symbol problem.
- A symbol means something but it can't be explained by a collection of simpler symbols. This is called the dynamic symbol problem.
- A symbol itself and the way people "interpret" the symbol are different. Usually due to some social aspect. This is called the social symbol problem.
FYI: This took a lot longer to write than I thought. I wish more people would write a laymans explanation of their scientific publications