Catch Everest Update | Customization

What exactly does customization entail, in regards to a theme update? "Please Note: Any customizations you have made to theme files will be lost. Please consider using child themes for modifications." I see there is an update for Catch Everest today. I don't believe I've done any major customization to my site, but am nervous to update if there are major changes on my site. https://ieo.wp.d.umn.edu/ Will I be affected or will any of my information be lost when I update the theme? Any info is greatly appreciated! Kind regards, sbartsch