Below the fold is an expression used to represent the portion of a webpage that the user cannot see on his, or her, screen upon loading. The area that you scroll down to see is considered below the fold. The "fold" can vary based on screen/resolution and devices so it is common practice to optimize web pages and landing pages for the most common screen sizes (resolution). To find the most common screen sizes used visit this resource on browser display statistics.
Don't forget to also check out: above the fold.