These indicators are so the teacher knows that the student was absent via one of two ways:

Absent when Assigned (which indicates the Student was Absent or Tardy when the Assignment was Assigned) or Absent when Due (which indicates the Student was Absent or Tardy when the Assignment was Due)

If the teacher is going through their grade book later and can't remember why the student didn't turn in the the assignment, they can easily see if they might have missed it because they were absent on the assigned or due date by hovering over the A icon in the Gradebook next to the Student/Assignment.

This does display also to the Student on the Students Progress Tab.