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 dialogue appears. Select your older original document and confirm the dialogue.
LibreOffice combines both documents into the reviewer's document. All text passages that occur in the reviewer's document but not in the original are identified as having been inserted, and all text passages that got deleted by the reviewer are identified as deletions.
You can now accept or reject the insertions and deletions. At the end you may save the reviewer's document as a new original with a new name.