The previous line of this poem means a pure thought.
According to a search on Sohu.com, we learned that this poem comes from "Poetry Thirty-Nine" by Pang Yun, a lay scholar in the Tang Dynasty: One thought is pure, and lotus flowers bloom everywhere. One flower and one pure land, one soil and one Tathagata. This poem shows the poet's empty-hearted and returning state of mind. The poet's Pang Yun was a Zen monk in the Middle Tang Dynasty. The layman's name is Yun and his courtesy name is Daoxuan. He is also called layman Pang.