@shubi imprinted is another way of saying marked, which means that the Omega has been marked by a certain alpha partner, by this, the Omega can only react to the pheromones of that certain alpha. Alphas can have multiple marked omegas, and, depending on the way the author wants it, sometimes marking needs to be done on a certain part of the body, like the neck, waist, back, etc...
however, in some other stories, being imprinted can mean to be a medical procedure done just for the Omega to not feel anything in particular by alpha pheromones, however I'm not really well informed in how the medical procedure it's done, anyway, I hope this helps you to understand it a bit better! (≡^∇^≡)