The concept of keeping the main message 'above-the-fold' goes back to when newspapers were first printed. Being broadsheets they were folded for sale – which meant that only the top half of the paper was visible to the passer-by. It quickly became clear that those papers with attention grabbing headlines on the top half of the page (‘above the fold’) sold more quickly.
This basic principle remains the same for digital content.
For your website, landing page, email or newsletter the ‘fold’ is the bottom of the screen. So make sure that your main messages and the call to action are readable without your audience having to scroll down 'below the fold'.
Keep your primary message (AIDA) above the fold
When briefing your website developer please note that not all screens are the same, plus with the proliferation of pads and phones, your website will present in many different ways. There are also screen resolutions and browser plug-ins to consider.
Talk to your web developer and make sure that all options are checked before the site is fully published.