A tsukubai (蹲踞) is a small basin provided in Japanese Buddhist temples for visitors to purify themselves by the ritual washing of hands and rinsing of the mouth (perform ablutions). This type of ritual cleansing is also the custom for guests attending a tea ceremony.
Tsukubai are usually of stone, and are often provided with a small scoop, laid across the top, ready for use. A supply of water is provided via a bamboo pipe called a kakei.
The tsukubai shown here, from the Ryōan-ji temple, is famous for the inscribed poem. The kanji written on the surface of the stone are without significance when read alone. If each is read in combination with 口 (kuchi), which the central bowl is meant to represent, then the characters become 吾, 唯, 足, 知. This is read as "ware tada taru (wo) shiru" and translates literally as "I only know plenty" (吾 = ware = I, 唯 = tada = only, 足 = taru = plenty, 知 = shiru = know). The meaning, "what one has is all one needs", is meant to reinforce the basic anti-materialistic teachings of Buddhism.