So Google has a premium version of their Google Analytics that costs $150K a year. One of the tools available is multi channel attribution.
Think of it like this. Someone searches on Google for something and they find your site. Look at it then leave. Then a few days later they see an ad on another site. Or maybe they are reading an article somewhere else and they link to your site. The user clicks that link, and purchases something for you (or signs up for your newsletter).
So now that visitor is now a loyal reader of your site, and Analytics tells you that is because of referral traffic. But what about the ad? What about the fact they found out by searching and found you on Google? They helped in the conversion too, right?
Giving credit to multiple channels along the way is considered multi-channel attribution. And that wasn't part of the free version of the GA tool, until now... :) #googleanalytics