score:8
This passage is according to the abhidhamma treatment of the attainment of nibbana. The two to three mind moments (yes, that's what it means) are called anulomañāṇa ("anuloma~naa.na") - knowledge of conformity, the twelfth stage of knowledge.
The Visuddhimagga (XXII.128) describes this according to the abhidhamma:
128. As he repeats, develops and cultivates that equanimity about formations, his faith becomes more resolute, his energy better exerted, his mindfulness better established, his mind better concentrated, while his equanimity about formations grows more refined.
129. He thinks, “Now the path will arise.” Equanimity about formations, after comprehending formations as impermanent, or as painful, or as not-self, sinks into the life-continuum. Next to the life-continuum, mind-door adverting arises making formations its object as impermanent or as painful or as not-self according to the way taken by equanimity about formations. Then next to the functional [adverting] consciousness that arose displacing the life-continuum, the first impulsion consciousness arises making formations its object in the same way, maintaining the continuity of consciousness. This is called the “preliminary work.” Next to that a second impulsion consciousness arises making formations its object in the same way. This is called the “access.” Next to that a third impulsion consciousness also arises making formations its object in the same way. This is called “conformity.”
(Path of Purification)
The next thought moment arising after these three takes nibbaana as an object.
EDIT: In plain terms:
Through the gradual understanding that all formations are impermanent, suffering, and non-self, the meditator attains an absolute certainty of one or another of the three characteristics and this leads to a release based either:
This realization lasts two to three thought moments, during which the mind realizes the truth of suffering and discards the truth of the cause.
The next thought moment is an experience of cessation, where there is no arising of sense experience (including mental sense experience). This is the realization of path consciousness (the noble truth of the path) because it has the power to destroy fetters and therefore lead to freedom from suffering. It takes nibbāna as an object.