Given that Frost's poem "The Secret Sits" is a simple couplet (two lines of poetry) and written in anapest trimeter (anapest- a meter with a three syllable foot; trimeter- the existence of three feet within a single line of poetry), the meaning of the poem is left up to the reader.
We dance round in a ring and suppose,
But the Secret sits in the middle and knows.
In regards to what Frost could be saying about the world we live in, one could interpret the poem using religious thought. We, meaning Frost is including the reader in the action, spend our lives dancing in a circle while contemplating life. In the center lies a greater power, the "Secret" is God. Only God knows the truth behind all which exists. "We" can only try to imagine the meaning.
Another interpretation of the poem, in regards to the world we live in could be based upon the fact that life is left to interpretation. With each individual, they are given the right to interpret the world as they see fit based upon their own experiences and morality. That being said, we are simply allowed to "suppose" about the true reality of life. The "Secret" lies in the center, it is the only one who knows the truth about life. No one on the outside will ever gain access to the secret because it will forever remain an unknown- as denoted by the proper name "Secret".
Basically, Frost is saying that mankind can only circle the reality of what life really is. No man/woman can enter the truth of the world because we all have our own understandings of how to define the world which we inhabit.