तस्मादज्ञानसंभूतं हृत्स्थं ज्ञानासिनाऽऽत्मनः। छित्त्वैनं संशयं योगमातिष्ठोत्तिष्ठ भारत ॥

tasmād ajñāna-sambhūtaṁ hṛit-sthaṁ jñānāsinātmanaḥ chhittvainaṁ sanśhayaṁ yogam ātiṣhṭhottiṣhṭha bhārata

Therefore, with the sword of the wisdom of the Self cleaving asunder this ignorance-born doubt dwelling in thy heart, be established in yoga. Stand up, O Bhârata.

In plain words: Krishna tells Arjuna to use the wisdom of the true self like a sword to cut away the doubt in his heart. That doubt comes from not understanding. Then Arjuna should stay steady in yoga, and stand up and act.