If you have not heard of wireframes before, then in basic terms they are a very simply layout of a web page that includes no colour, no imagery and, in some cases, no text. They can be sketched out or done digitally, either way it is very important to create these skeletons for a future website design.
When a designer first starts a web design project, they will look at the list of features and content that needs to be included in the site. This helps the designer to create a visual hierarchy with those elements on the websites page. It also helps for clients because once they have viewed the wireframes they are able to tell which elements are more important than the others.
Furthermore wireframes determine the layout with the user experience (UX) in mind. By leaving the creative design elements like the colour, imagery, and other details, until later on, you can focus on the layout and functionality of each element on the page to ensure the structure needed for the best user experience is created.