Above the Fold
What does above the fold mean?
The idea goes back to early publishing days when newspapers had to catch the eye of passing customers when their papers where folded in half on displays.
It's the prime real estate that is the first thing visitors (and search engines) see when they land on your page.
Where is the "fold" on a website?
The short answer is it's the content that appears on a website before the user scrolls.
There is no black and white answer here on how big that section is as many people use multiple devices, with different screen sizes, every day.
As a general rule of thumb you could say that the fold line is approximately 1,000 pixels wide and 600 pixels tall. However, the reality is you should go and look in your analytics and where you can see the real results of users that are already on your website, and design pages specifically for them.
How does above the fold content impact SEO?
Google created the Page Layout Algorithm in January 2012 to lower the rankings of websites featuring too many ads in this space. They said it would impact less than 1% of the web.
In October 2012 this search engine algorithm update went through a revision (as almost all of the updates do). This gave websites that had been pulled down by the initial launch to make a recovery. At this point Google came out with an update that around 0.7% of English queries were being affected by this.
Then again, in February 2014, Google released another refresh of their Page Layout Algorithm.
Then, in more recent years (November 2016) Google said this algorithm is now pretty much realtime and doesn't need manual refreshes or updates to be made by them. After crawling your pages they would pick up changes automatically and adjust rankings.
You may be thinking "this is old news" but this will continue to be an important factor. Again, in March 2017 Google came out and confirmed the Page Layout Algorithm is still a big deal and will continue to be important as improved user experience is a primary goal.