On the compare page you'll find a component that looks like this:
This is Happo asking you to manually review the diffs on the page and either accept or reject them. By accepting, you signal to others that the diffs are expected, or at least okay from a design standpoint. The component will then switch to this:
By rejecting, you signal to others that the diffs aren't okay, and that the author needs to follow up with something.
If you have the right CI setup, the accept/reject status will also be posted back to the PR/commit on github.com.
In rare cases you will see an alert message at the top of the compare page informing you about updates made. This is Happo informing you about changes made to internal infrastructure that might affect the screenshots and cause diffs. These could be e.g. browser updates, internal bug fixes, etc. You'll only be alerted about changes that were made in the timespan between when the "before" report and the "after" report were created.