what does imprint mean?

Imprint is kinda like Amore at first sight only way.. stronger. When they imprint they have to have the person they imprint on in their life some way no matter what. Even if the are just friends, its not really necessarally romantic.

it means to fall deeply in Amore with like a soul mate "for life"

A werewolf (or shapshifter) will imprint on the person who is most likely to carry on the werewolf gene and produce the best offspring. Imprinting can only occur once the lupo gene has triggered and they start phasing. They will imprint once they set eyes on the imprintee and that person will be the most important person in their whole world and they will be whatever their imprintee needs (a brother a friend a best friend and finally a lover) and they will do anything for them.
This idea is very similar to that of some animals, such as baby ducklings- in psychobiology imprinting is a form of learning in which a young animal fixes its attention on the first object it has visual, auditory o tactile experience. This is similar as the lupo would be the young animal (they are young werewolfs, not young humans) and the imprint occurs when they fix their attention on another through a visual experience

sorry for the long explination, hope i helped