it can vary a bit from author to author, but typically getting marked means you've been mated with (or claimed by) the alpha. ideally it's done consensually as a symbol of commitment between an alpha and omega who love each other, but if an alpha does it without the omega's consent (like is being implied here with this cliffhanger) then that is staking a claim to the omega that can make it impossible for them to be with another alpha. i've read media where doing that means the omega is physically repulsed by any other alpha's scent, or other alphas won't come near the omega (cuz they respect another alpha's claim more than the omega as a person...but that's a different can of worms). so i really hope su-ah didn't actually bite juhyeok especially since juhyeok clearly has not come to terms with being an omega much less his feelings for su-ah :(
Well, being repulsed and all that is how Juhyeok is rn bc of Su-ah's pheromones. Something to add, being marked (depending on the author) is similar to an intimate moment, it needs after care, depending on the author the Omega can become incapacitated to do anything w/o the alpha's help for a phew days, or weeks. The Omega can't spend a lot of time w/o the alpha, they feel each other emotions and, depending on the author, they can feel each other's pain.

i want to believe suah has a nosebleed or he bit himself to stop himself from biting juhyeok but he's had the spiral eyes this entire chapter so it's not looking good for him</3