धृत्या यया धारयते मनःप्राणेन्द्रियक्रियाः। योगेनाव्यभिचारिण्या धृतिः सा पार्थ सात्त्विकी ॥

dhṛityā yayā dhārayate manaḥ-prāṇendriya-kriyāḥ yogenāvyabhichāriṇyā dhṛitiḥ sā pārtha sāttvikī

The unwavering firmness by which, through Yoga, one restraineth the activity of the mind, of the life breaths and of the sense-organs, that firmness, O Pârtha, is pure.

In plain words: Pure firmness is a steady willpower built through yoga practice. It keeps the mind, the breath and the senses under control.