Matthew Henry really talks about this well! Check it:

Ps 91:1
He that dwells, (that sits down), in the secret place of the Most High, shall abide under the shadow of the Almighty; (he that by faith chooses God for his guardian shall find all that in him which he needs or can desire).

Note:
1. It is the character of a true believer that he dwells in the secret place of the Most High; he is at home in God, returns to God, and reposes in him as his rest; he acquaints himself with inward religion, and makes heart-work of the service of God, worships within the veil, and loves to be alone with God, to converse with him in solitude.

2. It is the privilege and comfort of those that do so that they abide under the shadow of the Almighty; he shelters them, and comes between them and every thing that would annoy them, whether storm or sunshine. They shall not only have an admittance, but a residence, under God's protection; he will be their rest and refuge for ever.
(from Matthew Henry's Commentary on the Whole Bible: New Modern Edition, Electronic Database. Copyright © 1991 by Hendrickson Publishers, Inc.)

Psalm 91:1