तमस्त्वज्ञानजं विद्धि मोहनं सर्वदेहिनाम्। प्रमादालस्यनिद्राभिस्तन्निबध्नाति भारत ॥

tamas tv ajñāna-jaṁ viddhi mohanaṁ sarva-dehinām pramādālasya-nidrābhis tan nibadhnāti bhārata

But Inertia, know thou, born of unwisdom, is the deluder of all dwellers in the body; that bindeth by heedlessness, indolence and sloth, O Bharata.

In plain words: Krishna says dullness comes from not knowing and fools everyone. It ties people down through carelessness, laziness, and too much sleep.