The terms are a bit whack in this manhwa for some reason, but what they just did is usually referred to as marking, where the alpha bites down on the omegas scent gland causing their pheromones to mix together/infuse. It usually means that the omega will only ever go into heat for that alpha, and will become repulsed by everyone else. Depending on the author it varies as to if omegas can mark or not. It's kinda like the omegaverse version of marriage.
(・o・)
It's generally creating a bond. Japan has it as a one way deal where an Alpha just bites the nape of an Omega to bind them together. However Korean has a more balanced approach to their binding method that isn't so animalistic or one-sided. Still it depends on the author and culture that determines how their characters deepen their bond.
In regards to this manhwa, the MC is describing what's going on with his body as his boyfriend initiates the imprinting, and as you can see it conveys two people coming together on an instinctual level.
What's imprinting?