When you open an article that is already being edited by another user you get a warning message telling you that someone else is working on the same document and that there are unsaved changes.

From here you can choose to:

• Wait until later to edit the article.  
  If you want to do this, click Cancel and come back at another time.
• Try again, to see if the user has since closed the document. 
  If the document is still being edited, you will get the same message.
  If you wish to contact the person who was editing the article and to ask them if they are finished and if so will they please save the document the Opened By information tells you who is currently editing the article.
• Or you can ignore lock and proceed with editing which overwrites anything the other user has done.  
  Be aware when you do this that other author can also save over your work. The person who saves last is the one whose article becomes the final article.