The relevant passage in the SEP is:
Anyone who inclines to realism, but also follows Augustine in confining consciousness to the momentary present, faces the difficulty of explaining how we can have an awareness of succession if our consciousness consists of nothing more than a succession of momentary snapshot-like experiences. Kant solves this problem by offering a richer account of these momentary states of consciousness. In the visual case, momentary episodes of visual experiencing are accompanied by representations of recently experienced visual contents. More generally, these representations (or retentions) allow us to be aware that our presently occurring experience is a part of an ongoing process.
Your snapshot of consciousness is Augustines 'confining consciousness to the momentary present'. But then the question is how do we then perceive change, that is succession. Further down in this entry of the SEP, you have cinematic realism which does roughly what you might think it does. Cinema after all consists of static frame after static frame but when we watch it we are only aware of movement & not stasis. But the authors say that this position suffers from defects, and not seriously considered.
What they propose is that momentary states of experiencing is not all we experience in the moment, we also 'experience' its immediate past. They call this representation - as it is not immediate experience, its in the past after all, and they also call it retention as its part of our memory - we retain the experience of the past.
It is obviously Kantian - in the sense of talking about the unity of consciousness from the unity of experience (from the unity of phenomenal reality). That is the unity of visual consciousness from the unity of visual experience of the unitary phenomenal world.