Yoga Sutras 1.51
Patanjali, explained by Swami Vivekananda
तस्यापि निरोधे सर्वनिरोधान्निर्बीजः समाधिः ॥
tasyāpi nirodhe sarvanirodhānnirbījaḥ samādhiḥ
By the restraint of even this (impression, which obstructs all other impressions), all being restrained, comes the "seedless" Samadhi.
In plain words: When even this last impression is stilled, everything in the mind is at rest. Then comes "seedless" samadhi, where no seeds of past actions remain and the true Self is seen.
Swami Vivekananda explains
You remember that our goal is to perceive the Soul itself. We cannot perceive the Soul, because it has got mingled up with nature, with the mind, with the body. The ignorant man thinks his body is the Soul. The learned man thinks his mind is the Soul. But both of them are mistaken. What makes the Soul get mingled up with all this? Different waves in the Chitta rise and cover the Soul; we only see a little reflection of the Soul through these waves; so, if the wave is one of anger, we see the Soul as angry; "I am angry," one says. If it is one of love, we see ourselves reflected in that wave, and say we are loving. If that wave is one of weakness, and the Soul is reflected in it, we think we are weak. These various ideas come from these impressions, these Samskaras covering the Soul. The real nature of the Soul is not perceived as long as there is one single wave in the lake of the Chitta; this real nature will never be perceived until all the waves have subsided. So, first, Patanjali teaches us the meaning of these waves; secondly, the best way to repress them; and thirdly, how to make one wave so strong as to suppress all other waves, fire eating fire as it were. When only one remains, it will be easy to suppress that also, and when that is gone, this Samadhi or concentration is called seedless. It leaves nothing, and the Soul is manifested just as It is, in Its own glory. Then alone we know that the Soul is not a compound; It is the only eternal simple in the universe, and as such, It cannot be born, It cannot die; It is immortal, indestructible, the ever-living essence of intelligence.