Before a person reads your resume, software does. It opens the file, pulls out whatever text it can, and tries to work out where your last job started and ended. If your layout has two columns, the extractor often reads straight across both of them, so a job title ends up welded to a skill from the sidebar. If your contact details sit in the page header, some parsers drop them entirely. If your icons come from a symbol font, they arrive as unreadable characters sitting in the middle of your phone number.
None of that is visible to you, because on your screen the file looks exactly as you designed it. So this tool shows you the other version: the plain stream of text extracted from your own file, in the order it is stored. Most people find at least one thing they did not know was happening.
Then it rebuilds the same content as a single column document with real text and standard headings, and you can edit anything that came across wrong before you export it. Your file is read in your browser and never sent to a server.