What it means: Any document that is built a different size than the final trimmed size of the magazine. All pages should be built to the actual trim size of your magazine. So, all pages for an 8 3/8″ X 10 7/8″ magazine should be built to 8 3/8″ X 10 7/8″.
Problems caused: Pages built to the wrong size will have a variety of bleed problems and/or text or other elements that will be trimmed off.
How to fix: Change the document size in your page layout program to match the final trim size of your magazine and adjust elements on your pages accordingly.