Wireframes, this is new word for me because in my previous works I have not heard and seen about it.
It is very useful for tester and developers to understand and analyze the requirements. As definition, Wireframes are a basic visual guide used in web design to suggest the layout of fundamental elements in the interface. As in testing domain, testers develop the some test documents before deliver the code on middle environment. So for develop the test documents testers need to understand and analyze the requirements. Mostly testers get the requirements in word format but if testers get the requirements in visual format also then it will give more and more understanding about the requirements.
For more the details about Wireframes go though this link http://en.wikipedia.org/wiki/Website_wireframe