O que significa 세요 ?

See a translation

it's imperative/indicative ending with subject honourification. (-시- + -어요 → 시어요 → 셔요/세요)

드세요=
들다(honourific verb for 'to eat') + 시어요 → 드 + 시어요 → 드셔요/드세요 "(please) eat"
ex) 맛있게 드세요
polite toward someone lower: 들어요 ex) 먼저 들어요 (please eat first) 들다 + 어요 (polite imperative)

여기 보세요 (look over here here please)
보다 (to see) + -시- + 어요
→보 + 시어요→보셔요/보세요

지금 오고 계세요 "(someone higher than me) is coming" "he/she/they is/are coming(verb with subj. hon.)"

가세요 as in 안녕히 가세요=
가다 + 시어요
→가시어요→가셔요/가세요

뭐 하세요? what are you doing?
하다 + 시어요→하시어요→하셔요/하세요

in 해요체 (-아/-어요 ending. informal, moderately addressee-lowering speech), -아/어요 looks the same in plain ending, question ending, commanding ending…