A black rectangle drawn over a PDF often hides nothing: the text is still underneath, and copy-paste gives it back. Redact turns every page with a redaction into an image with the boxes painted in, so the covered text, fonts and vector shapes are gone from the file, not just out of sight. Pages without redactions are kept as they are, minus annotations, unless you flatten every page.
The rest of a flattened page's text is put back as an invisible layer, so the file can still be searched. Any run of text that touches a box is left out whole, which can remove a little more than the box covers (a label on the same line, say). The Verify tab shows exactly what's left, page by page, and warns when a word you searched for is still in the file. Metadata like the author, title and the app that made the file is stripped. Images are redrawn and saved as a new PNG, which drops EXIF and GPS data.