'Set Selected as Completed' is a feature that allows Teachers to manually finalize student's work. Whether they forgot to hit submit or time is up, choosing this option will essentially force turn-in your student's work. This will allow you to review your students work whenever you're ready instead of spending time chasing down overdue assignments. 

To use this feature, select the checkbox next to student's work that is still "In Progress", choose the 3-dot menu in the upper right and select "Set Selected as Completed. You can also do this for an individual student submission by choosing the 3-dot menu to the right of their submission information and choosing "Set as Completed."

This is also useful when the "Submit" button has been disabled to prevent students from accidentally submitting their work meaning you will have to turn it in for them.