A diff shows exactly what changed, and nothing else. Instead of comparing two versions of a file by eye, you see the removed lines and the added lines side by side or one under the other.
In Codex, every change comes as a diff so you can review it before you keep it. You don't need to understand every line. Check three things: are the changed files the ones you expected, is the change about the size you expected, and did the tests pass?