Stands for "Automatic Document Feeder." An ADF is used in copiers and scanners to feed pages into the machine. It allows multiple pages to be copied or scanned at one time without the need to place each individual page in the copier or scanner.
A flatbed scanner, on the other hand, requires each page to be individually placed on the scanning surface. While this is convenient for single pages, copying 80 pages would require a lot of manual page-loading! Some copy machines include both an automatic document feeder and a flatbed scanning surface so that either method can be used.