Popularized by Oli Gardner of Unbounce, the term has to do with the number of distracting elements on a landing page. More specifically:

The Attention Ratio is the ratio of interactive elements (links) on a page, to the number of campaign conversion goals (which is always one)

The main point of this term is that the more links you have on a landing page, the less likely a conversion goal will occur. Conversion goals are typically form submissions or button clicks. Whatever the goal is, you are less likely to have that event to occur with each additional outbound link that you add to the page.

Here’s a page that Oli Gardner offers in this entertaining guide:

Noisey Landing Page

Wherever there is orange on this page, there is a link. In this case, the attention ratio for this page is 56:1. That’s because there are 56 separate links that the visitor could click other than the desired call to action. In order to see an improved performance on this page, it’s always best to lower the attention ratio as much as possible (by decreasing the number of links).

It’s as simple as that!