In Lisp, macros are powerful language constructs that allow programmers to define new language abstractions and extend the language itself. Macros enable the transformation and generation of Lisp code at compile-time, providing a way to create domain-specific languages (DSLs) and custom control structures. Here's an explanation of what macros are and how they work:

1. Definition and Expansion :
   * A macro is defined using the `defmacro` special form in Lisp.
   * The `defmacro` form associates a macro name with a template that specifies the desired code transformation.
   * When a macro is invoked in code, it undergoes a process called macro expansion.
   * Macro expansion involves the substitution of macro calls with the code specified in the macro template.
   * The macro template can contain Lisp expressions and special constructs to generate code.

2. Compile-time Code Transformation :
   * Unlike functions, which are evaluated at runtime, macros are expanded at compile-time.
   * During the compilation process, Lisp's macro expansion mechanism is invoked to transform the macro code into equivalent Lisp code.
   * The expanded code is then compiled and executed as part of the program.
   * Macro expansion allows for powerful code transformations, enabling the creation of new language constructs and abstractions.
3. Code Generation and Abstraction :
   * Macros provide a way to generate code dynamically based on the macro arguments and the desired code transformation.
   * By manipulating Lisp expressions, macros can generate and transform code to achieve specific goals.
   * Macros enable the creation of custom control structures, domain-specific languages, and abstractions tailored to specific programming tasks.
   * Macros help improve code expressiveness, readability, and maintainability by allowing developers to define higher-level abstractions that match the problem domain.

4. Runtime Efficiency :
   * Macros can help improve runtime efficiency by performing computations at compile-time rather than runtime.
   * By generating code specific to a particular use case, macros can avoid redundant calculations or unnecessary runtime checks.
   * Macros enable optimizations and code transformations that can lead to more efficient execution.

5. Metaprogramming :
   * Macros enable metaprogramming, which is the ability to write programs that manipulate or generate code.
   * With macros, Lisp programmers can extend and modify the language itself, adding new syntax and semantics as needed.
   * Metaprogramming with macros empowers developers to mold the language to fit the requirements of their applications and solve complex problems in elegant ways.