Attribution modelling is all about acquisition channels – that is, where those clicks came from that lead to the final conversions.
Your acquisition channels can be online, like with your direct traffic, referral traffic, your advertising, PPC, organic search and all that jazz. They can also be offline marketing channels, such as ads you put in newspapers or any ads you manage to get on the TV.
There can be a great deal of acquisition channels before a conversion is made. I found this infographic about just this which I think can explain things a little better than I can.