Website traffic comes from a referral from another website.
That referral could be a link from another website, like a blog or even non hyperlink link on say a video.It could be for bought traffic like Homegain, or banner ads. Or it can be the motherload of all traffic the search engines. The traffic from an engine can come from more paid ads, or from organic search.
Organic search happens because an engine reads your page(s) and thinks it knows what you page(s) is about. And It found your page because it followed a link from somewhere.
Now the debate rages on weather or not that link should be similar to, or themed to your page that it is pointed at. The answer is no. As Jonathon Leger points out it isn't necessary. Although it makes sense that it should be themed.
However I believe someday as the engines get better and better, they logically will want it to be themed. Thus, it is a pretty good idea to do the themeing now.
On the same subject, how do they know it is themed? Well there is this idea of trust and I am not so sure that is easy to do right now as defined in the paper. But certainly, the text around a page and the title is the most obvious indicators to an engine on weather or not the link page is similar to the page it points. Another great equalizer that I have seen making a huge difference is age. Age of page and age of link and the page the link is on.