Child Theme

At http://documentation.hb-themes.com/highend/#child-theme it states “If you modify an existing theme and it is updated, your changes will be lost.” It also states “You need to use a Child Theme only if you are making code customizations.”

Does this mean that changes to code on the page will be lost, or does this mean that changes to code off the page (a CSS file for example) will be lost. To explain further, all I’m doing is changing the appearance of pages by adding and removing elements.