In Google Analytics, the Source/Medium dimension provides information on how traffic was brought to your site. Each channel has two pieces of information: a source and a medium.
A source is the actual origin of your traffic –like a specific search engine (outlined in blue).
A medium is the more general category the specific source falls under, like CPC ads (outlined in red).
In the example image, Google is the source of the traffic, and Cost Per Click (cpc) ads are the medium.
Information from the Source/Medium column helps determine which sources are driving more traffic and which sources are falling behind. This helps diagnose issues with certain advertising mediums, and concentrate time and money where they are needed.