Who's who 사람
Sunbae
선배 seonbae
A senior: someone who started before you.
A sunbae is someone with more experience in the same field. Age does not decide it, the debut does. A singer who debuted earlier is a sunbae, even when they are younger.
The polite form is sunbaenim (선배님). The opposite is hoobae.
They bowed to their sunbae before the stage.