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 red while the added ones are in green. All lines in which changes occurred are highlighted in grey.
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: