And it came to pass that as my father arose in the morning, and went forth to the tent door, to his great astonishment he beheld upon the ground a round ball of curious workmanship; and it was of fine brass. And within the ball were two spindles; and the one pointed the way whither we should go into the wilderness...And we did follow the directions of the ball, which led us in the more fertile parts of the wilderness. 1 Nephi 16:10, 16

Nephi wakes up and finds brass ball/compass outside his father Lehi's tent.  Inside the ball were two spindles/pointers that directed him where to go.

This "magic compass" guided them through the wilderness and where to find food.  It also had writing on it.

And it came to pass that the voice of the Lord said unto him: Look upon the ball, and behold the things which are written.  And it came to pass that when my father beheld the things which were written upon the ball, he did fear and tremble exceedingly, and also my brethren and the sons of Ishmael and our wives.  1 Nephi 16:26, 27

It's unknown what was written on it, but it scared them.  How did the magic compass/ball work?

And it came to pass that I, Nephi, beheld the pointers which were in the ball, that they did work according to the faith and diligence and heed which we did give unto them. 1 Nephi 16:28

The compass only worked according to the faith of the people who used it.  If they had faith, it pointed to food.  Does this sound too had to believe?  Another book in the Book of Mormon, Alma, tries to clear things up.

And now, my son, I have somewhat to say concerning the thing which our fathers call a ball, or director—or our fathers called it Liahona, which is, being interpreted, a compass; and the Lord prepared it.
And behold, there cannot any man work after the manner of so curious a workmanship. And behold, it was prepared to show unto our fathers the course which they should travel in the wilderness.
And it did work for them according to their faith in God; therefore, if they had faith to believe that God could cause that those spindles should point the way they should go, behold, it was done; therefore they had this miracle, and also many other miracles wrought by the power of God, day by day...And now I say, is there not a type in this thing? For just as surely as this director did bring our fathers, by following its course, to the promised land, shall the words of Christ, if we follow their course, carry us beyond this vale of sorrow into a far better land of promise.  Alma 37:38-40, 45

• The compass is now called Liahona (a word that scholars struggle to find its meaning in Hebrew, now the title of a Mormon magazine).
• No man could make it.  God did.
• Belief powered it.
• The compass brought them to the Promised Land.

In the Bible, when God directed his people through the wilderness to the Promised Land he used himself.  God led them with pillar of smoke by day and fire by night.  God acted as the compass.  He spoke through Moses to take them to the food.  God did not need a magic compass.

The idea of mysterious brass balls and compasses is something found in the mind of fantasy writers and treasure hunters.