When you edit revision X of some page and somebody else decides to do the same then you might run into problems when you save the page. The website tries to merge both changes into the page. Often enough this works without problems.
When the two of you edited the same part of the page, however, the second person to save the page will run into an edit conflict. In this case, text saved will include conflict markers.
Conflict markers look like this:
<<<<<<< you lines you wrote ||||||| ancestor the original lines ======= lines the other person wrote >>>>>>> other
Using these markers, you can find the text where you had a conflict and resolve it.