Given two version (old and new) of a Git change (i.e., individual commit or patch series from a pull request) it produces a diff that is actually useful for reviewing purposes, assuming you've already reviewed the old version of the change.
It's sort of like `git range-diff`, but where `git range-diff` produces a "diff of diffs" that is very hard to impossible to read, this tool gives you a direct diff between old and new versions, but filters out any irrelevant changes that were introduced because the author rebased on a more recent version of the target branch.
I hope that makes sense - I never know quite how to put it into words for somebody to understand who isn't intimately familiar with Git. It is very powerful though if you combine it with a minimal amount of setup e.g. for fetching all PR branches from a GitHub repository. I use it almost daily as part of my code review workflow.