Every Yarrly is actually made up of four 'bits'. Two pictures comprising the first and second panels, and then two bits of (optional) text.
Every Yarrly created is remixable. This means that if you see a Yarrly and open it in Yarrly (see this article for information on opening a yarrly in Yarrly), you can tap the wheel and choose REMIX from the popup menu.
Once you hit remix, the Yarrly bits will be downloaded to the phone. You can then change some or all of the elements of the Yarrly and then create it.
This creates a new remix of the Yarrly you edited. Remixes of a Yarrly create a yarrly chain - a 'thread' or sequence of Yarrlys based on an original Yarrly and multiple remixes - perhaps by lots of different people.
The creator of the original Yarrly and anyone else who has contributed to the Yarrly chain will get a notification (a yarrbottle) telling them a new remix is available to view.