Chunks are simply er... chunks of plain text or HTML, and are inserted as-is into a document being processed. If there are any MODx tags in the chunk content, they will be processed during the next pass through the parser (a maximum of 10 passes in all). They are often used for basic content, such as footer or header content, that will be used on all pages, even if the pages use different main templates. This way you only have to change the address or phone number in your page footers, for example, in the chunk, and the change will work across all pages no matter what template they use.
Chunks are also useful if you need to have a very long, complex snippet call with lots of parameters, and need to use the rich text editor on the document. The rich text editor can "helpfully" totally trash the snippet call’s parameter list. So putting the snippet call in a chunk, then using the chunk tags in the document content will protect the snippet call.
http://sottwell.com/how-templates-work.html
Wayfinder, Ditto and a number of other snippets are coded to use given chunks as mini-templates for the various parts of their output. Usually these mini-template chunks will contain HTML code and [+ ... +] placeholders, which are taken from an array that MODx uses to store output or other values that snippets can produce for later use.