Source is the origin of traffic, such as Google, Bing, Facebook, a newsletter, or a referring website.
It matters because analytics turns marketing activity into decisions. Without clear measurement language, teams may misread performance, overvalue the wrong channels, or miss the actions that actually drive revenue.
Example
A visit attributed to a particular newsletter has that newsletter as its source when the link is tagged accordingly. Pair source with medium to distinguish email distribution from another channel that uses a similar source name.