Activities are locked in the gradebook to stop grades from being altered. When an activity is locked it is impossible to enter or change a grade. This has two main purposes:
It is often used by professional staff around exam board, to prevent changes while grades are externally reviewed and transferred into Student Two
It is used a defacto cut-off date for University-wide activities like the academic integrity quiz, Library World and Pre-Place - these activities don't have a traditional due date, so locking the grades stops late submissions from being recorded in the topic
In the Gradebook, click on the Setup tab, then the Gradebook setup link
For the item you wish to change, click the Edit link to the right of the assignment and select Edit settings
Tick the Locked box
Alternatively, if you click on the Show more... link, you can set a date when the activity should be locked. However, it is important that you do not have both options in use simultaneously
If you need to unlock the Gradebook, follow the above steps but uncheck the locked box.