What is meant by above the fold?

The page area of any website that is first loaded and visible on the screen, without scrolling down is "Above the fold"
fold basically means, the upper part of a page is visible and the lower part hidden.
Above the fold is a popular term nowadays for SEO and refers to any data which we have added on a website visible to a user on load without scroll

It's like 'above the fold' was a term used for content that appeared on the top half of the front page of a newspaper.
When you open a website the first thing you see is consider as above the fold. This feature is also know is one of the main factor of getting good response of your visitors.