When a user opens a file for editing, a special security feature within Mambo locks the file in a "Checked Out" state. When a file is locked, the only person who can modify it is the user who checked it out. This prevents problems where two people might be trying to modify the same file - naturally enough, different versions of the same file cannot be saved at the same time! So, the locking of the file prevents data corruption or loss.

Now, this is an extremely useful feature, but it is important that all Mambo users understand how it works.

A file remains checked out and locked until the user either cancels or clicks "Save". When that happens, the file gets checked in and is once again available for editing by other users.

But my content item was saved so why is it still checked out?

If you have a content item open for editing and you navigate away from that content item, get a connection problem, or close the file without saving it or cancelling it, the content item will remain checked out.

One of the most common reasons a content item will save modifications but stay locked is the "Apply" button. The "Apply" button applies changes you have made to the file. If you are doing a lot of editing on a content item, or you are writing a long article and want to come back to it later without worrying about someone else messing with your article, the "Apply" button makes sure you don't lose your changes.

However, when you have finished modifying the file you MUST use the "Save" button to tell Mambo that you have finished and let it check in the item.

I didn't open a content item, but it shows up as checked out!
If you want to edit an item that is displayed as being checked out, you need to contact the last author who checked out that item and ask them to check it in. Sometimes, content items are still checked out because someone is still working on them (they may be having a break, coming back to their editing another day, or whatever).

What is the Global Check In?
Mambo has a feature that allows an Administrator to check in all files that are checked out, thereby unlocking them all and allowing anyone to edit or otherwise modify them.

To access this feature, you go to System -> Global Check In from the Administrator backend. You will see a list of all the files that are currently checked out. After doing a Global Check In, Mambo will return a list of all the items that were checked back in and unlocked.

Use this with care!
If you run a Global Check In while someone is working on a checked out item, their work will be lost.

Summary:
To recap..

You open an item for editing, it becomes locked in the checked out status.

You Cancel, the item gets checked in and unlocked.

You Apply your changes, the item remains checked in and locked.

You save the item, it gets checked in and unlocked.

Remembering these few things will mean you won't need to post on the forum saying, "Help! I can't edit my content".