In plain words
That firmness, never parted from yoga, by which one holds back the functions of manas, prāṇa, and senses from straying is sāttvikī, Partha.
Word for word
The unfailing firmness by which one holds back, through yoga, the workings of manas, prāṇa and the senses, that firmness, O son of Pṛthā, is sāttvikī.
धृत्या यया धारयते मनःप्राणेन्द्रियक्रियाःयोगेनाव्यभिचारिण्या धृतिः सा पार्थ सात्त्विकी
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam· bhāṣya
धृत्या यया — अव्यभिचारिण्या इति व्यवहितेन सम्बन्धः, धारयते ; किम् ? मनःप्राणेन्द्रियक्रियाः मनश्च प्राणाश्च इन्द्रियाणि च मनःप्राणेन्द्रियाणि, तेषां क्रियाः चेष्टाः, ताः उच्छास्त्रमार्गप्रवृत्तेः धारयते धारयति — धृत्या हि धार्यमाणाः उच्छास्त्रमार्गविषयाः न भवन्ति — योगेन समाधिना, अव्यभिचारिण्या, नित्यसमाध्यनुगतया इत्यर्थः । एतत् उक्तं भवति — अव्यभिचारिण्या धृत्या मनःप्राणेन्द्रियक्रियाः धार्यमाणाः योगेन धारयतीति । या एवंलक्षणा धृतिः, सा पार्थ, सात्त्विकी ॥ ३३ ॥
In plain words
By this steadiness — and notice, the word 'unwavering' from later in the verse really belongs with it, even though other words come between — a person holds the movements of mind, breath, and senses in check. Holds back what, exactly? The restless activity of mind, breath, and senses: held by this steadiness, they simply stop wandering onto paths scripture doesn't sanction. 'Through yoga' means through concentration; 'unwavering' means sustained without a single break. In short: with unwavering steadiness, through concentration, a person holds the movements of mind, breath, and senses in check. That, Partha, is the steadiness that belongs to sattva.
Close to the Sanskrit
By that resolve — the connection, though separated by intervening words, is with 'unswerving' — one holds up; what? — the activities of mind and life-breath and senses, that is, mind and life-breath and senses, mind-life-breath-senses; their activities, their movements — those, from proceeding on a path outside scripture, one holds back, restrains — for, being restrained by resolve, they do not become matters of a path outside scripture — by yoga, that is by concentration, unswerving, that is, accompanied by constant concentration — that is the meaning. This is what is said: by unswerving resolve, through yoga, one restrains the activities of mind, life-breath, and senses that are being restrained. That resolve which has this character, O Partha, is sattvic.