This refers to Moodle 3.6 from July 2019 onward.
The Post Date is when grades are released to students. It should be on or after the Due Date - usually after, giving enough time for marking to take place.
If anonymous marking is enabled, when the Post date and time for an assignment is reached, then the names of the students become un-anonymised, so the Post Date should be set to be after the end of the marking period.
When using Feedback studio, "Post Date" is when marked assignments will become visible to students. After the Post Date, students will be able to access both grade and feedback from you through the blue pencil icon.
More information here
If you are having trouble with students not able to see their grades:
The following other resources are also relevant to this topic: