This help page describes how ‘Pages not found’ works as a metric. See the Pages not found dimension for more information.
The ‘Pages not found’ metric shows the number of hits where a dimension was set or persisted at the time a visitor encountered an error. This metric is valuable when you want to see which pages or URLs contained error messages (such as a 404). You can then pass this information on to your web development team, who can determine the cause of the error and fix it.