Answer:
The statement means that the small nation is home to many diverse cultures, civilizations, and tribes who all call it home and grow up, co-existing amongst themselves in harmony.
Explanation:
The statement that "Nepal is a common garden of many castes, cultures, and civilizations" means that the country is like a garden that houses a variety of tribes and cultures without any specific tribe or caste being allowed to reside alone. The statement is a metaphor, comparing Nepal with a garden that is made up of mixed varieties of tribes and cultures.
Though small the country may be, it is home to many tribes, many different cultures living peacefully, co-existing, and growing up together. It is a peaceful ground for the numerous cultures to grow up and develop, at the same time co-existing amongst themselves. Without a single tribe, a single civilization, or a single culture dominating or living in it, the complex yet mixed nature is what makes this small country a great example of secularity.