If you're not the only one with editor rights to a page, it's possible for you and someone else to edit the same page at the same time. For this reason, it's to your advantage to save frequently, and to break large documents up into small sub-pages, reducing the chance of an editing conflict.
When you save your edits, if someone else has also changed the page, QDL merges those changes into your page, and saves the result.
If both of you edited the same paragraph, QDL won't know how to merge the changes together. In this event, a pop-up window will inform you that there is a conflict, and in the saved page, you'll see something like this:
******* BEGIN YOUR VERSION *******
I know more about basketball than anybody.
******** END YOUR VERSION ********
******* BEGIN THEIR VERSION ******
I really like football.
******** END THEIR VERSION *******
It's up to you to re-edit the page, determine how to weave the edits together, and save the page with the conflicts resolved.
Note that if the conflict occurred inside HTML tags (such as a change in a URL inside anchor tags, where everything else stayed the same), the conflict markers might not display properly. In this case, you'll have to use the feature, to edit the HTML manually.