As unknowable as the garden's maker is its meaning. Many Japanese gardens display two smaller rocks flanking a larger one to signify Buddha and two disciples. Other configurations spawn other meanings. The garden at Ryoan-ji originally bore the label "Tiger Cubs Crossing the Sea," but no one knows why. A more general interpretation takes the gravel for the sea and the rocks for islands. At this point, no one seems to care.
One fact has struck everyone as significant: It is not possible to see all 15 rocks from any single position along the viewing deck. Perhaps only when full illumination comes, through meditation, will all be visible.
Read more of Willard Spiegelman on the zen garden at Kyoto.