Can be used as a question or statement of who is or should be next or aks the question of what's coming up.
This phrase is an old shipping term. It was used to find out who was or was supposed to be on guard, doing duties, etc. This term later moved into the realm of baseball and is still used to this day as a description of who's going to be batting next.
"Let's get this line move'n people. Who's on deck?"