A wireframe is a used to show a general layout of a site. It helps to graphically depict placement of the various elements on a web page (header, content, imagery, forms etc), all of which allows the designer to achieve consistency in design and aids with rapid prototyping. The wireframe becomes a crucial part of development as it enables clients to familiarise themselves with the site and navigaiton at a top level and from the earliest stages of a project.
Having a wireframes helps build a graphical communications between clients and agencies, with many clients wanting to see the site visually. Also, clients are able to decide which factor they like (and don’t like) about the site before building, saving a lot of time and headaches at the build stage.