The seven main aesthetic qualities in Zen 禅gardens are asymmetry, simplicity, patina, naturalness, massiveness or deepness, freedom from convention and tranquility.
Another way to describe this idea in Japanese is “Wabi Sabi and Shibui 侘寂渋い”
It is about imperfection, impermanence and transience. It represents asymmetry, asperity, simplicity, modesty, intimacy and suggestion of natural process.
Specifically for the garden design, the idea is called “Karesansui 枯山水“or using stones and pebbles to represent mountains and moving water. Arrangements of such suggest asymmetry, simplicity and transience.
One would see ocean and mountain valleys in the arrangements, and some would find much deeper meanings of life and transience of our existence.