They only show the rough skeleton of an interface.
Wireframes are a design approach that only shows the skeleton of a user interface. The idea is to present the overall concept without getting lots in details.
Therefore wireframes can be considered part of the low fidelity prototyping phase when they are evaluated in an e.g. paper prototype together with users.
There are no contents and details to be seen
The idea of wireframing is to present first concepts and approaches to a problem UX Designers want to solve. A wireframe is far away from a finished product, software or service and therefore details like images, texts, labels, detailed forms and similar content do not play an important roll at all. The overall concept is in the focus.