Psalm 91 vs 1 says “He that dwelleth in the secret place of the Most High shall abide under the shadow of the Almighty.”

Where is the secret place?

Given that the psalm was written in the old testament, likely by Moses in the wilderness, could the secret place have been in the wilderness? obviously not.
Also, given that at that time, the holy of holies was where Shekhinah (God's presence) appeared which only the priest was allowed to visit once in a year, it is definitely not a building or a terbernacle.

It's a real place but not a physical place, not a geographical location. The secret place is not a church. You do not have to come to church to be in the secret place of the most high. You can be in church and not be in his secret place. It is not even your prayer closet, however precious your secret times of prayer may be. That’s not what the Psalmist meant when he said, he that dwelleth in the secret place of the Most High. 

Is it a mental/spiritual location?

It’s not a shrine, and it is not a meeting point. It’s not a country, not even a holy land. It is not a denomination, and it is not a doctrine.

The secret place of the Most High is the heart of God, the place of faith in God, love for God, confidence in the love of God in Christ, obedient trust in the mercy of God. It is a state of heart. It is our state of heart in the heart of God. And we may enter it and we may abide in it and we may be in it or out of it. But the secret place of the Most High is there for us.

It's a secret place

It is a secret place and it is secret, not because it is hard to find, nor because it is hidden or difficult. It is a secret place because there are so few who enter it. It is an open secret, it is secret because it is unknown to anyone who lacks spiritual perception/eyesight and has no faith in God. There are a few hungry, eager people who are seeking the secret place of the Most High and are finding it.

The secret place is a place of safety and security for those that seek shelter in the LORD, it is a place of silence and solitude.

Psalm 90:1 (KJV) says “Lord, thou hast been our dwelling place in all generations”.

So the secret place of the most high is not a one-day affair, it's a lifelong endeavor.

The psalm ended with “Because he loves me,” says the Lord, “I will rescue him; I will protect him, for he acknowledges my name. He will call on me, and I will answer him; I will be with him in trouble, I will deliver him and honor him. With long life I will satisfy him and show him my salvation.”

To dwell in the secret place of the Most High means to love God, to live with a constant awareness of God’s presence in your life. It is to put your complete trust in God to protect, help, and guide you on a daily basis as you face life’s challenges. It’s remaining calm in the face of apparent hostility and knowing that God has your back.