Source is one of the four main dimensions (along with medium, campaign and channel) for reporting and analyzing how people found your website. Source tells you where the message was seen. For example, a source of ‘google’ would indicate that someone found your website after performing a search on Google. Source can be used in combination with medium for more granular insights, for example, a source of ‘google’ and a medium of ‘cpc’ would be reported for paid clicks from your AdWords campaigns.