The review function is available in LibreOffice for text documents and spreadsheet documents.
Imagine you have some co-authors or reviewers who collaborate with you writing your original document. One day you send out copies of your document to all reviewers. You ask them to edit the copy and send it back.
Normally, the reviewers enable change tracking by Edit - Track Changes - Record and you can easily see the changes.
If one of the authors has made changes to a document without recording them, you can compare the changed document to your original document.
Open the reviewer's document and then choose Edit - Track Changes - Compare Document.
You should always start with opening the newer document and compare it with the older document.
A file selection dialog appears. Select the copy of the document and confirm the dialog.
LibreOffice combines both documents into your original document. All text passages that occur in your document but not in the copy are identified as having been inserted, and all text passages that are missing in your original document are identified as deletions.
You can now accept the "insertions", in which case the relevant texts stay in their original form, or you can accept the "deletions", in which case the marked text contained in the copy is not inserted in your document.