What does "Check-in" mean and do in Joomla?

I noticed the the Check-in lock image ( ) show several times, and always after getting logged out due to timeout.  When you move your mouse over the lock without clicking, it shows the above type of message.  I've seen this lock show up on modules, and articles.  So, what does this do and why does it occur?  It is done to prevent problems. In the example of an article that is being worked on, the article gets locked when being worked on, so other Admins cannot edit it or worse at the same time as you.  When you get logged out, for whatever reason, that article could remain locked. and won't be available to be worked on until it is checked in.  Basically, when a module or article is being worked on, it gets checked out.  It is sort of like checking out a DVD or book at the library, and the checkedout items needs to be returned to the library and scanned back in (Checked-in) to be available again for checkout.

This is derived from the techniques used by a team of programmers working on the same software.  To know when an item is being worked on and avoid conflicts of overwriting someone's work, a check-out and check-in process was and is commonly used.

Doing a Save or Cancel closes the article and checks in the article and thus available for modification by an authorized user.  Note that doing an Apply does NOT check-in (does not close) an article, but does save the changes.

Please note that I do not get paid to write these articles.  All are done in a best effort manner.