The Change History screen gives a list of any changes within your environment such as schema changes or database parameter changes within the time range selected. The list can be filtered, sorted or exported to CSV. You can also register other change events that aren't detected automatically. This is useful for tracking code releases or infrastructure changes for example. These can also be added via the API.
Change detection runs as a background process that is scheduled every 5 minutes by default so the actual change time could be anytime within the last 5 mins of when the change was detected. This means that sometime the change marker might not line up exactly with the performance impact of the chance. If you would like to run change detection more frequently see the FAQ here.