This can have 3 reasons: 1. most commonly when you submit your timesheet for the current (ongoing) period. Once you submit you're not supposed to change the timesheet and so it's locked. An admin can unlock the timesheet however. Read more… 2. you're looking at a day that is (far) in the past. How far you're able to go back and edit is a setting that your organization controls, so you can ask an admin. 3. as an admin you're looking at a user's timesheet, it's locked by default, but can be unlocked to make changes.