Yoga Sutras 3.55
Patanjali, explained by Swami Vivekananda
तारकं सर्वविषयं सर्वथाविषयमक्रमञ्चेति विवेकजं ज्ञानम् ॥
tārakaṃ sarvaviṣayaṃ sarvathāviṣayamakramañceti vivekajaṃ jñānam
The saving knowledge is that knowledge of discrimination which simultaneously covers all objects, in all their variations.
In plain words: The highest knowledge is the power to tell the real from the unreal. It sees all things in all their forms at once, not one after another. It is called "saving" because it frees a person from the cycle of being born and dying again and again.
Swami Vivekananda explains
Saving, because the knowledge takes the Yogi across the ocean of birth and death. The whole of Prakriti in all its states, subtle and gross, is within the grasp of this knowledge. There is no succession in perception by this knowledge; it takes in all things simultaneously, at a glance.