[This isn't a Wicca question, but ok]
What eye and chakra are the same question, your third eye located in the center of your forehead. Do not focus on opening just your third eye chakra, it deals with insight, creativity, and clairvoyance, but each chakra needs to be open and equally balanced to be beneficial. One that's too open or closed will result in the energy pooling and causing negative effects. A third eye which is too open can cause migraines and some say hallucinations.
Various theories are around to explain why someone can or can't speak with spirits, genetics, strong spiritual connection, third eye. Some theorize because children and animals have different eye structures they are more aware but as humans age our eyes worsen so we can't see spirits anymore. And of course many people want a buck and make up stories of being able to communicate with spirits/demons.