An attribution model is the framework that determines how you attribute credit for a conversion in web analytics. In other words, do you give credit for a sale to the last ad that a consumer saw before they made a purchase? Or do you attribute a sale to the first ad that a consumer saw, that first exposed the consumer to your product. Google Analytics, for one example, is setup by default to attribute conversions to the “last interaction” or final touchpoint before a conversion or a sale took place. There are many ways to create a custom attribution model that assigns different weightings to touchpoints that may occur along the way to a sale or conversion.
As a side note, last click attribution tends to favor search engine marketing, because many “searchers” are searching for products because they are prepared to immediately buy them. In contrast, first click attribution tends to favor display advertising, because even though most customers don’t buy something immediately after they are exposed to a display ad, they may convert at some point in the future.