It`s a small lightweight application of Circle of Fifths. In music theory the circle of fifths (or circle of fourths) is the relationship among the 12 tones of the chromatic scale their corresponding key signatures and the associated major and minor keys.
At the top you have the key of C major which has no sharps or flats in its key signature. Each stop on the circle as you go clockwise from C is a key with one more sharp than the previous key. Each stop as you go down counter-clockwise from C is a key with one more flat than the previous key.