The impersonal passive can be used when you want to describe an action, but you don't want to specify who did it, and there was no logical object to turn into a subject for the passive. Amongst the commonest examples is itur ad Rhenum (in a story about the past), "*it was gone to the Rhine → one went to the Rhine". We usually pick a personal pronoun instead of "one", when available and logical in the story, e.g. "we went to the Rhine".
It is often difficult to translate into Germanic languages. In modern Romance languages, I think you do this by using a reflexive pronoun: on se voit, "one sees oneself", usually "we see each other", where se is "oneself".
I would classify it as somewhere between active and passive, a bit like the Greek middle voice. Note that, even in personal constructions, Latin can sometimes use the passive to express a middle sense, but this is uncommon. It is common only in impersonal constructions.
So in your example, I think "we proceed" or "the text proceeds" would be fine translations.