Attribution is assigning credit for a conversion. In last-click attribution, all of the value associated with the conversion is assigned to the last marketing activity that generated the revenue.
However, there are other attribution models that can help you better understand the value of each of your channels. For example, rather than assign all of the value to the last channel, you might want to assign all of the value to the first channel, the one that started the user on the customer journey. This is called first-click attribution. Or, you might assign a little bit of value to each of the assisting channels in the customer journey.
Source: Google Analytics