The ‘Overlap Campaigns’ option is used to display the campaigns that have some overlap.
For campaigns to appear here, they should be scheduled to play on the same days (at least one day should be overlapping).
You can check this option out with clicking on the ‘Overlap Campaigns’ buttons in the ‘Campaigns’ entity. Below is an example when no Campaigns are overlapping:
When some campaigns are scheduled to overlap at least one day, they will show up in a list in this modal window, with some further information such as the Channel and Campaign names and the Start and End dates.