The third eye (orMind's eye, for the tumblrites that cry"appropriation" to call it that) can be used to see spirits and the astral planes. You won't see them as if they're physically in front of you, but rather as an image in your mind. You know they're there, because the image isn't forced and just..appears.
Some people are born with the ability to see spirits, others call them. In my family, we mostly communicate with spirits using their ashes, if they were a loved one. However, I am able to call them whenever I need guidance. Before I began to call certain spirits I wanted to communicate with, I would see or feel their presence.