With my limited understanding this is what I think it means:
With the practices of Yoga one ultimately reaches a state where the (1)Purusha/human being performing the yogic practices[The Knower], (2)the yoga practices themselves [The instrument of knowing] and (3)and the object of meditation which is GOD/UNIVERSE/Higher intelligent power [The known] all become one. We ultimately reach a state where there becomes no distinction between (1),(2),(3) and that state of attainment is known as Samadhi.
Once the human mind reaches that state of samadhi , the consciousness is said be in Samapatti. What the state of samapatti is I’m really not sure but it is explained as follows,
“Like an object reflected flawlessly in a clean mirror, the perceiver, the perceived and the instrument are reflected as one. This transparent reflecting quality of conciousness is termed samapatti, which means assumption of the original form of the seer.”
So I think this means Samapatti is the state wherein (1)/(2)/(3) are reflected as ONE which is supposed to our true original state of being.
Once again what the state of Samapatti/Samadhi as explained above is my limited theoretical understanding of it, which is open to each one’s own interpretation.
The state of Samadhi and Samapatti is said to be attained by the regular practice of the 8 limbs of yoga-Yama’s, Niyama’s, Asana, Pranayama, Pratyahara, Dharana, Dhyana, and Samadhi.
Explanation as given by BKS Iyengar
With refinement, the conciousness becomes highly sensitive, choiceless, stainless and pure. The perceiver, the instrument of perception and the perceived object, clearly reflected, are nothing but the seer. Like an object reflected flawlessly in a clean mirror, the perceiver, the perceived and the instrument are reflected as one. This transparent reflecting quality of conciousness is termed samapatti, which means assumption of the original form of the seer.
Patanjali’s description of samapatti underlines the subtle distinction between yoga, samadhi and samapatti. Yoga is the employment of the means to reach samadhi. Samadhi is the profound meditation, total absorption. Samapatti is the balanced state of mind of the seer who, having attained samadhi, radiates his own pure state. Yoga and samadhi, in other words, are regarded as the practices; samapatti the state towards which they lead.
-Excerpt from ‘Light in the yoga sutras of Patanjali’ by B.K.S Iyengar