If you are working on an article item and leave it without saving your changes, the item becomes locked. This will prevent someone else from accidentally making changes to that item before you are finished with it.
Help my Articles are locked and I know no one else if working on them
1) This padlock symbol tells you that a page is locked
How to unlock these pages
1) From the top menu visit Tools > Global Check-in
Revisit your articles – they will now appear all unlocked
Preventing Locked Articles
Make sure you save or close an article before leaving it, if you dont the item will become lock to prevent others from editing that item.