What is 卯木 (mǎo mù) in Chinese metaphysics?

Yin Wood of the due east (Zhen palace), commanding mid-spring; hides Yi — flowering woods and grasses, signifying kindness and grace.

Original (Chinese): 正东震宫阴木，居仲春之令，藏干乙木，为琼林芳草，主仁慈秀美。