Tethering flights is the ability to associate two or might flights within a campaign together. This will allow you to push the start/end dates of multiple flights at once.
To Tether multiple flights press "Command" on your keyboard + click the flights first flight you want to tether. Then repeat this will all flights you would like to associate together. To push the date ranges of these flights press "Command" and right click on your mouse.
Tethered flights will have a light blue shading:
Tethered menu (Command + right click on mouse):