If you are trying to work on a page on your Joomla website and it's saying that it's locked by another user, then you will have to first unlock it before you can work on it. This happened because someone else is actually working on the page or else they closed their browser without first getting out of the page by clicking Save or Cancel.
Here's what you do:
First, it's best to make sure that nobody is actually really working on that article / page. You don't want to stomp on their work. They may be working on that page right as you are trying to get access to it. Call them or ask them if they are in it. Check the date that is displayed and see if it has been checked out recently or not.
Second, if you know they are not in it, then the lock can be removed. To do this, you will have to be a SuperAdministrator type of user. Also, when you do this (or when the Super Admin does it), know that all locks on all pages will be removed.
Third, the SuperAdministrator will go to this menu item:
Tools > Global Check-in
and then all page locks will be removed.