Wireframes are visual guides representing a webpage's structure, used in website and app design. They outline the page layout, navigation, and interactions without focusing on design elements like colors or graphics. Wireframes serve as a blueprint for the design team, ensuring all elements are considered, including content placement, features, and functionality, promoting effective usability and user experience in the final design. They can be low-fidelity, hand-drawn sketches or high-fidelity, digital mock-ups, helping teams collaborate and establish the basic design structure before detailed design and development.