The side-by-side diff view displays different versions of the file - the older one and the newer one - together on one screen. You can inspect the entire content of both versions and compare the differences line-by-line.
The deleted fragments are highlighted in grey while the added ones are in green.
If the revisions contain comments, you can display them by clicking the Comments icon at the top left:
You can open the side-by-side diff view from the revision view by clicking the corresponding button in the upper-right corner of the inline diff pane: