A wireframe displays the content of a page as well as a preliminary display of the navigation and functionality. It is used to help define a web page. Wireframes are also an important reference doc for designers to know the contents of a page or template before starting the design. Note, while wireframes do display a page layout, they shouldn’t be considered the design. It’s best to not limit design possibilities with wireframes.
Wireframes are critical to understanding what will be required to design and build the page and consequently the website. They are a key deliverable of a Definition Phase.