What is the significance of the collar?

probs that she's an omega

I think it’s his old family wanting him back since he recognized the collar

they are probably a family where they sell their omega to high rabking families

In omegaverse a collar is used typically by omega to protect themselves: basically on the back of their neck is a mating gland, that if bitten by the alpha during their heat will make them a "marked" omega, basically they'll be mated through that bite making the omega technically the "property" of the alpha. It's also regarded as a sign of deep love and trust between partners, and in most omegaverse manhwas or mangas, the omega can also claim the alpha through a bite on the ear (you won't find this in every version btw). Add to that the fact that their heat and ruts will align to give them higher chance to conceive, as well as the fact that the omega will not need to put a collar on anymore js to make sure no alpha bites him due to his pheromones since they will be blocked for everyone, but his mate.