From Carleton Moodle Docs
Date after which the grades will be locked. Usually this date is set to the end of the activity, and the beginning of the grading process.. A typical sequence of events would be:
|Start of activity and participant submissions||No||Yes|
|End of activity and beginning of grading/feedback||Yes||Yes|
|End of grading/feedback, and release of grades||Yes||No|
Effects of locking
Locking prevents changes to grades. It disables the "quick grading" and "quick feedback" features of the Grader report, and prevents modules from updating grades. Locking affects grade items and grades differently, as outlined below.
In grade categories
Locking has no effect on the category itself, it is only used to affect a large number of grade items at once (bulk action), which in turns locks all their grades. The locking effect cascades down to all the category's children. If the category contains categories, the locking effect continues to cascade down, locking all categories, items and grades below the initial category. A grade item can be unlocked even though its parent category is flagged as locked.
In grade items
A locked grade item doesn't accept changes to its settings or grades from its related module. When there is no related module (as for manual grades), the locking simply prevents grader report users from changing the grade using the grader report interface.
If a grade item is locked, its grades are also locked, and cannot be unlocked until the grade item is also unlocked.