BEM, or Block Element Modifier, is a methodology used in web development and design to create reusable and maintainable code. It helps developers structure their CSS classes in a way that makes it clear how different components of a webpage relate to each other. The "Block" represents a standalone component, the "Element" is a part of that block, and the "Modifier" indicates a variation of the block or element. By following the BEM convention, developers can avoid conflicts in styles and improve collaboration within teams. This approach enhances readability and makes it easier to understand the purpose of each class. Overall, BEM promotes a systematic way to build user interfaces, especially in large projects.