In Act I of Our Town by Thornton Wilder, George and Rebecca, brother and sister, are introduced. George is a prominent character who marries Emily, Mr. Webb's daughter. George and Emily want a marriage that epitomizes "happily ever after," and in a way they succeed.
In Act I, Wilder introduces an ingenious literary device that works as foreshadowing and as the adhesive that thematically holds a seemingly random series of events together. Rebecca and George are in conversation, however the audience only hears a small, seemingly meaningless, snippet of it. Rebecca says,
"I never told you about that letter Jane Crofut got from her minister when she was sick…on the envelope the address was like this: Jane Crofut; The Crofut Farm; Grover’s Corners; Sutton County; New Hampshire; United States of America....Continent of North America; Western Hemisphere; the Earth; the Solar System; the Universe; the Mind of God—that’s what it said on the envelope…And the postman brought it just the same."
George replies: "What do you know!...What do you know!" Act I ends. Act II opens with the Stage Manager introducing the morning of George's marriage to Emily.
In Act III, George throws himself down in tears on Emily's grave and here we finally learn the reason for and the meaning of Rebecca's seemingly nonsensical contribution in Act I. The address on the letter is two things. It is the foreshadowing of the early death of Emily in the reference to the "Mind of God." It is also the vehicle for one of Wilder's themes: the transcendence of the soul of humankind over the trivialities and mundane activities of daily life on Earth.
So, what it means literally is that working from the smallest unit--Jane Crofut--outward to the greatest unit--the Mind of God--the individual person is a component of something very big and very important, making the individual important and big, too. What it means as a literary device is that there will be within the play a merging of at least one character--and one related to George, the hearer of the lines--with the Mind of God, which occurs through death for the character. What it means thematically is that which has already been said, each individual transcends the mundane and trivial because each is part of a greater whole that has similarly great and significant parts--the Mind of God.