For a submission to be viewable by a student, that submission must be finalized. We recommend finalizing submissions as you finish grading them; that way, you can track how many submissions still require grading.
However, you might want to finalize all submissions at once. For example, you might have forgotten to finalize a few submissions along the way, and just want to finalize everything before publishing an assignment.
Good news: it's possible to finalize all submissions in a few clicks. Navigate to the Assignments Overview page, and select "Bulk edit. From here, you can finalize all of your submissions. (You can also unfinalize them all in one go.)