Marketing attribution is the process of measuring all touchpoints (traffic sources) that participate in customer journeys and then determining which touchpoints drive conversions.
Ideally, a customer directly buys your product the first time they visit your website. Realistically, there are several touchpoints involved before a conversion is actually made. For example, a customer might read about you on a blog, is retargeted via display, pays another direct visit to your site and finally converts after clicking on an ad on Google.
Determining the value of each traffic source is something to think about. Each traffic source plays a role in your sales process, and therefore deserves credit for it. How much exactly is credited to one source or the other, is regulated by an attribution model. These are frameworks for determining which traffic sources - touchpoints - across the customer journey receive a certain amount of credit for a conversion. Different attribution models distribute the value of a conversion between touchpoints in various ways.