LLD (Low-Level Design) for backend systems involves translating the high-level architecture into detailed components, modules, and interactions. It focuses on defining the data structures, algorithms, and interfaces required to implement the system's functionalities efficiently. LLD diagrams, such as class diagrams or sequence diagrams, are commonly used to illustrate the relationships between different components and their behaviors, ensuring a comprehensive understanding of the system's design and facilitating the development process. Additionally, LLD documents serve as a blueprint for developers to follow during the implementation phase, promoting consistency, scalability, and maintainability in the backend design.