Fear imprint stages are just phases in the puppy's life when he is more sensitive, or tuned, to learning what he should fear.
There are two main fear imprint stages: the first (which is kind of the topic of the OPs question) occurs after 8 weeks of age, usually around 10 weeks. This is the point in a young pup's development when he is (or would be, in a natural/wild environment) gaining more independence from his mother. So he needs a little information about what he should be afraid of and give a wide bearth to.
The second fear imprint stage occurs in adolescence. This one is much more intense and longer lasting. Again, it has a developmental reason - this would be the age that adolescents start to be involved in the more dangerous pack activities such as hunting. It is vital that the young dog understands what he needs to be frightened of.
In both cases (though again, it is usually FAR more apparent in the second fear imprint stage) the dog learns by first being afraid/hanging back/avoiding - and then
watching his mother or other more senior members of his pack (or in modern day pet environments - YOU) for information on whether there is really a need to be afraid. When he establishes that there is or isn't - that belief will stay with the dog for life (or at least, would require a lot of convincing for him to change his mind

).
Naturally, any bad or frightening experiences a pup has during one of these phases is going to have a far greater impact than if the same event happened outside of the fear imprint stage. That is why its generally not the greatest time to take a pup away from his mother (or to schedule his early vet visits) as you need to work quite hard to ensure that the experiences are all fairly positive for him.