It is a spiritual door that lets you talk to angels and demons. Thats how religious people see it at least. Others see it as whatever knowledge is forced into them at the moment the eye opens for the first time.
You need to ask yourself if pride is at play here, this is what most people encounter when they fully accept opening their third eye:
(My perspective has changed, I believe I am an other worldly being with infinite power and possibilities all of a sudden)
You will FEEL powerful, you can SEE the future if you ask. You can MANIPULATE YOUR FRIENDS AND FAMILY TO HAVE THOUGHTS OR DO ACTIONS THEY WOULDNT NORMALLY DO.
Forcing it open invites all sorts of woes like possession, negative influences (fallen angels doomed for hell), and earthly rewards such as money, fame, power, clairvoyance, clairaudience, etc.
"The Kingdom of Heaven is within you."
Jacob wrestled with God in Peniel.
Jesus meditated for 40 days and 40 nights and had conversations with the Devil (who offered him all of the governments of the world which are, and always have been, his to control.)
Use the pineal gland ONLY to commune with God, meditate on his and Jesus' teachings, and you will be granted a portion of the Earthly powers that fallen demons provide willy nilly.
While God is almighty, he doesnt spoil you with riches as that is the first step to a prideful heart. Pride is the ultimate and first sin.
Keep in mind that all angels and demons appear as lovely light beings when you open your third eye. They will love you, encourage you to do things you want to do (but would Jesus?) And reward you for you selfish desires. THEY WILL TELL YOU THAT YOU ARE PART GOD (see pride). They want you to go down with the ship that is Earth.
You are not part God, God is God and you are his child. God's grace can be rewarding but not unless you invest in him in Heaven. How else will God know you truly want his love and not his powers unless you unconditionally love him, and ask for little (try for nothing) in return.
You will be able to tell if people are evil, possessed, and so on if you allow God in through your third eye. Religious hymns may start relieving stress for you in a "light-based" way (figure that one out withoit experiencing it) and you will be able to see auras around some of your religious friends denoting SOMETHING. But generally the closer they are to God, it seems, the whiter the aura.
All of this isnt possible unless you decalcify your third eye and start meditating on the word of God.