The teacher can view a list showing if or when students have opened the last grades that have been posted to the classroom:
Lists, for each student, when (or if) they have viewed their grades.
Whenever you upload a new grades file, the Grades button on student screens is marked with a red star and this list of who read their grades is cleared out.
Note that this shows only the time the student opened the last grades you posted. When you post new grades, the old grades are completely replaced along with any record of the students access to those grades.