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
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:
- signlessness - based on impermanence, that there is no telling what will happen in advance; e.g. the rising and falling of the abdomen suddenly speeds up;
- desirelessness - based on suffering, that there is no benefit to clinging to any formation; e.g. the rising and falling of the abdomen suddenly feels unbearable;
- emptiness - based on non-self, that all formations are void of self and there is no relationship of ownership or control in regards to all formations; e.g. the rising and falling of the abdomen appears to proceed automatically without interference from the meditator.
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.