If enabled, Allow submission from will restrict the student from submitting before this date. This will not affect their ability to enter the activity link and view the description, associated files or marking guide/rubric for the assignment.
Due date can be disabled if the activity is open for submissions for a length of time eg an assessment task that students build on weekly/over time.
No submission after date restricts students will be unable to submit after this date unless an extension has been granted.
Selecting this option means that the students will be able to see the activity description, even when the assignment is not yet open for submission.