What does "people are like Qiu Shu and their hearts are like wild horses" mean?

This is a metaphor, which refers to a person who looks gentle, intimate and calm on the surface, but is wild and ambitious inside.

Sentence: People are like Qiu Shu and wild horses, waiting for you to walk into my grassland.

Source: Preface to Heart Like a Wild Horse: Seven Essentials of Changing Worry and Cultivating Mind by Qiuyang Chuangba Rinpoche. Qiu Bai Rinpoche was born in Kang province, Tibet on 1939. At the age of thirteen months, he was certified as Master Zhu Gu (reincarnation guru). According to Tibetan tradition, the enlightened guru has the ability to reincarnate as a human being because of his sad wish. Such a guru may leave a letter or other clues before his death to predict the location of his next reincarnation.