In plain words
Know rajas to be of the nature of passion, the source of thirst and attachment; it binds the embodied one through attachment to action, son of Kunti.
Word for word
Know rajas [to be] of the nature of passion, the source of craving and attachment; it binds, O son of Kuntī, the embodied one by attachment to action.
रजो रागात्मकं विद्धि तृष्णासङ्गसमुद्भवम्तन्निबध्नाति कौन्तेय कर्मसङ्गेन देहिनम्
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam· bhāṣya
रजः रागात्मकं रञ्जनात् रागः गैरिकादिवद्रागात्मकं विद्धि जानीहि । तृष्णासङ्गसमुद्भवं तृष्णा अप्राप्ताभिलाषः, आसङ्गः प्राप्ते विषये मनसः प्रीतिलक्षणः संश्लेषः, तृष्णासङ्गयोः समुद्भवं तृष्णासङ्गसमुद्भवम् । तन्निबध्नाति तत् रजः निबध्नाति कौन्तेय कर्मसङ्गेन, दृष्टादृष्टार्थेषु कर्मसु सञ्जनं तत्परता कर्मसङ्गः, तेन निबध्नाति रजः देहिनम् ॥ ७ ॥
In plain words
Understand rajas as passion itself — the very word 'rāga' comes from a root meaning 'to color,' the way red ochre stains everything it touches. That's rajas: it colors everything it touches with wanting. It's born from two things together: craving, which is longing for something you don't yet have, and clinging, which is the mind's fond attachment to something you already do have. Both together give rise to rajas. And this is exactly how rajas binds a person, Kaunteya: through attachment to action — getting absorbed in, devoted to, actions whose fruits you can see and actions whose fruits you can't. That attachment to action is precisely how rajas keeps the embodied self bound.
Close to the Sanskrit
Know, understand, that rajas has the nature of coloring — 'rāga' from the root meaning to color — having the nature of coloring, like red ochre and the rest. It arises from craving and clinging: craving is longing for what is unattained; clinging is the mind's attachment, characterized by fondness, toward an already-attained object; arising from craving and clinging together — this is what 'tṛṣṇāsaṅgasamudbhavam' means. That, that rajas, binds, O son of Kuntī, by attachment to action: clinging, absorption, in actions whose results are seen or unseen — this is 'karmasaṅga'; by that, rajas binds the embodied one.