Wireframes are low to mid-fidelity screen designs that showcase layout and interaction. Taken as a whole, wireframes show workflow across the system. Each wireframe corresponds to a single screen, and often a single state of that screen. One screen thus can have many wireframes defining different states of the screen.
There always seems to be some debate in the UX design community around the need for wireframes, but the Official Fuzzy Math Take™ is that wires are an essential step in the UX design process. Wireframes help us focus on how a product will function to make sure that it works before moving into the visuals and development phases of a project.