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Macro is a convenient method to expand language constructs. When a macro is called, arguments are passed to the macro body, which is a macro expansion function, without being evaluated. Then, the macro expansion function expands the arguments, and returns the new form. The resulted form is then evaluated again outside the macro. It is an error to apply a macro or special form to a list of arguments. Macroexpand function can be used for the explicit macro expansion.
Though macro runs slowly when interpreted, it speeds up compiled code execution, because macro expansion is taken at compile-time only once and no overhead is left to run-time. Note that explicit call to eval or apply in the macro function may produce different results between interpreted execution and the compiled execution.