The phrase "to uncheck the box" means to remove or undo a selection or choice that was previously made by clicking on a small square or rectangle on a computer screen. Full definition
You don't need to wait until your 90 - day period ends, but only if you complete and submit this form (so don't use the usual method of unchecking the box for automatic renewal). (chrismcmullen.com)
You can deactivate automatic button swapping by unchecking the box next to Use Nintendo Button Layout. (tomsguide.com)
Consumers can choose not to install the recommended update by unchecking the box next to Windows Live Essentials 2011 QFE1 in the list of MU updates. (thewindowsclub.com)