Think of these as blueprints for your website. Usually created before any designs, these will show the general layout and functionality of your site. They typically contain no graphical elements and only placeholders for the final text and images just to give you an idea of how the website will be arranged. They are not the design.
Wireframes are an extremely important in the web design process and can allow for effective discussion over different user flows and page layouts. It is usually much easier to edit wireframes rather than changing things at the design and build stages.
Consider wireframing as the 'foundation' of your web project. Skipping on such a vital part of the process can have drastic effects on its success.