A snapshot saves the Overview as it is right now. Take one before you change something on your site, take another one afterwards, and compare the two. It is the easiest way to see whether a new theme, a caching plugin or an image optimizer really made a difference.
Save a snapshot
Go to the Overview tab.
Pick Desktop or Mobile in the header, and set the filter if you only want some of your pages.
Click Save snapshot and add a short note, like Before installing a cache plugin.
The snapshot keeps the scores, the distribution, the lab metrics and the top issues exactly as they were.
Compare two snapshots
Make your change, then recheck your pages. Tick Recheck all and click Start scan, or recheck only the pages you care about from the Reports list.
Save a second snapshot with the same report type and filter.
On the Snapshots tab, tick both and click Compare selected.
You see both snapshots side by side, with the change for every score and metric. Snapshots can only be compared when they use the same report type, so a desktop snapshot is compared with another desktop snapshot.