By creating a timesheet lock, an admin prevents users from adding, modifying or deleting their hours and comments in the specified date period. It's a good practice to lock a timesheet, you might for example miss any hours that are added after you billed a client.
How to lock a timesheet
Login as a user with administrative rights and go to Manage -> Locked periods.
The existing locks are shown in the left panel, ordered by start date. To create a new lock, modify the start and end date under "new lock". Everytime you change either the start or end date, the name is updated to reflect that change. You can always pick your own of course.
Press "Lock" to lock that period for all users.
Everytime you modify the start or end date of a lock you see an overview of the hours booked by who in that date range.
Click on any of the users to get a breakdown of the booked hours.
What the user sees
Users are still able to see their hours although it's read-only. A small "lock" icon is displayed to show why the hours are read-only.
In the screenshot on the right, January is locked but the user is still able to book hours in December.
You can do a quick report on the any locked period. In the report criteria screen there's a dropdown listing all locked periods, selecting one sets the start and end date of the report to that of the locked period.