Summary: A way of making pictures look good even if they are spanning across two magazine pages.
When you are creating a Word document - or any document, in fact - you're usually restricting yourself with keeping the content of a page to, well, a single page. In MagazineWorld™, however, designers have a whole load of cool little tricks they can use to make their designs leap off the page. One of them is to spread a design over several pages - this is known as a DPS, or a double page spread.
There are two main ways of keeping a magazine together. Saddle stitch (imagine taking a stack of paper, folding it in half, and putting staples in the spine - most comic books and newspapers are saddle stitched) and perfect bind.
When using perfect bind, you get a flat back of the book (this is also the technique used for most paperback books), but it also means that you can't open the magazine fully: you can't fold the magazine back on itself so only a single page is facing you, without damaging the spine.
Because of this 'valley' on the middle of the page, designers use 'split for spine', which means that part of the image is repeated where the page curves down into the spine, creating an overlap between the two pages. That way, the photo looks good despite being part of a perfect bound product, and creates the optical illusion that you're looking at a photo that isn't interrupted by the split down the middle.
The amount of splitting depends on the number of pages and the stiffness of the paper used in the magazine, and can vary from a few millimetres to about a centimetre.