Campaign attribution refers to how Google chooses which traffic source to give credit to when there may be more than one visit by a user.
Google normally tracks the most recent visitor source – for example:
If a visitor first clicks on an advert in the search results pages, then visits again later by typing in the brand name and clicking on the first natural (or organic) link, then the organic traffic source will take credit for the visit and any sales or goal conversions.
But if a user comes back directly – for example by typing in the full url, or clicking on a bookmark, then the previous traffic source will still take credit.
To stop later sources taking credit for sales and conversions, add ?utm_nooverride=1 to the end of any links in your adverts or email marketing campaigns.