All content is structured to some degree, since every CMS has some internal content model. However, to be called "unstructured" usually means that content lacks many datatyped attributes that an editor might otherwise expect it to possess.
For example, a Real Estate Listing that relied on editors to organize and format specific data in a rich text attribute, might be called "unstructured" (pejoratively, even). However, a simple Text Page content type might be equivalently unstructured and this might be appropriate.
The term is therefore often used to describe content that is less structured than other content or less structured than some arbitrary standard.