Matryoshka dolls are pretty cool. They're those Russian dolls, usually made out of wood, that have one big doll holding a bunch of smaller dolls inside it. You can get them in all sorts of designs, and indie darling Tim Schaefer even designed a video game around the concept.
Cloud Atlas's structure is based on this concept. Adam Ewing's story is the big mamma doll. Within it is Frobisher's tale, within that one Luisa's, and so on. Then we get to the baby of the story (although by no means the least important), Zachry's tale. From then on out, the dolls go back into one another. Zachry's leads back into Sonmi's, Sonmi's leads to Cavendish's, and so on, until the doll is complete again. Although the stories could stand alone, they are more than the sum of their parts. To understand the whole thing, you have to understand them all.
As if the matryoshka connection weren't apparent enough, David Mitchell also includes a few explicit mentions of matryoshka dolls. One of Ayrs's symphonies is called Matryoshka Doll Variations (2.2.3). Sonmi sees a carnival sideshow advertisement for "Madame Matryoshka and Her Pregnant Embryo" (7.1.184), and Isaac Sachs writes in his notebook that time is "an infinite matryoshka doll of painted moments" (9.41.8).
And that's the key: infinite. You can open the doll up and put it back together as many times as you want, and even though you can't see the future hidden inside the doll of the "present" it's always there, just waiting to come out.