Psychologically speaking, imprinting is when a younger
person (typically a child) follows in the footsteps of the first
person they interact with early in their life. It could be their
mother, big brother or big sister. When the child views that
person as their role model, they have essentially imprinted onto them. The said child fantasizes about being like their role model, being intimate with their role model (e.g. hugging), and willing to do anything they ask. This happens not only in humans but also in animals (e.g. baby chicks following and copying their mother
duck after hatching).
Going off of that, you could say that in general, imprinting is the act of looking up to a person and viewing them as their idol, often to an excessive and subservient degree.
Imprinting often happens when you crush on someone for the first
time. You just wanna be like them, be with them, and have
everything to do with them. When they're put in the same group as you, you suddenly feel a strong sense of devotion. That is the
essence of imprinting.