This is a calligraphy of the chineese character "Wu" ( in japaneese "Mu" ). It's meaning is literaly "No" or "Has not". But it is also a famous Zen koan. This is how it goes:

Joshu's "Mu"

A monk once asked Master Joshu: "Has a dog the Buddha Nature or not?"
Joshu said, "Mu!"

Master Mumon's poem:

The dog! The Buddha Nature!
The Truth manifested in full.
A moment of yes-and-no:
Lost are your body and soul.

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