The lock image indicates that someone is currently editing the article or that the last person who opened the article did not close it. If you hover over the lock you will see the date, time and the name of the person who last checked out the article. If it has been a while or you know that no one is currently editing the article, you can check it back in by clicking the lock image.

To prevent this problem you must hit "Save & Close" or "Close" when you have finished editing an article.