Whenever I open the document, it gives me an error message that links/sources are missing; what does this mean?

InDesign is looking for the files for your images; if it can’t find the actual file on the computer, this message will be generated. It will also lead to a low resolution preview on PDFs. It’s important to keep all files together when working in InDesign, but you’ll still be able to see the images in InDesign fine either way.