Material Design is Google's set of guidance and reusable pieces for designing digital interfaces. It includes recommendations for things like color, layout, buttons, and how parts of an app respond.
Think of it as a shared design kit that helps a team make screens feel consistent. A designer can start with familiar components and adjust them for the product.
Using Material Design does not mean every app must look alike. Teams can adapt it to their needs, and still need to check that their interface is clear and comfortable for their users.