When an editor opens an article to modify it, the other participants to the site who navigate to that article’s page are warned and are advised to open the article later in turn after the current editor is finished.
In fact, if two editors open the same article at the same time, the modifications that one editor makes risk "overriding" the modifications made by the other.
If you see the warning notice "Warning - an editor is working on this article", then you are strongly advised not to modify it at the moment. Come back to this article later, once the other editor has made and saved his modifications.
Conversely, when you yourself are working on modifying an article, other site participants who visit that page will be pre-warned that you are working on it. So long as you are modifying the article, and for a period of one hour, the other editors are invited not to open the article for modification. When you think that you have finished working on that article, and that others might want to take their turn at it, then you can "release" that article. The warning to the other participants will disappear, and they can then make their own changes to the document should they so wish.