It is a virtual garden where you can keep plants that you have collected. It is an alternative, non-competitive environment. All Zen Garden plants have to be watered (except aquatic plants, as they live in water) and fertilized to grow and either bug sprayed or played music to make them drop extra seeds periodically, but nothing negative will happen if you ignore their needs. While you can see the zen garden you seem incapacitated to everyone around you.

There are 32 available spaces for Zen Garden plants in the main Zen Garden, and eight slots in each of the other two gardens: the Mushroom Garden for plants that sleep during the day and the Aquarium Garden for aquatic plants. 

Seeds dropped by plants have the same stats as the plant which dropped those. Seeds you collected appear in your hands after leaving the zen garden. You can't plant seeds achieved through this way in the zen garden, but you can plant them anywhere they can grow. The first time they get in contact with water they grow:

small plants grow instantaneous to full size

medium plants grow in 1 round to full size

big plants grow in 3 rounds to full size 

enormous plants grow in 1 minute to full size

Notes: 2 times per long rest you can move one full-grown Plant out of the zen garden in your environment by touch, as long as the chosen plant can live in the environment. For example when you an aquatic plant can't live in the forest. If the environment is to small for the plant it depends on the plant and the environment whether and how the plant grows. For example a tree can break through glass but will be broken in a cave. It cost one action to move a plant this way., Zen garden, Requires Attunement - True, Attunement Description - by a druid, bard, sorcerer, or wizard