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These preprocessing directives are declared in the C++ Standard the following way

 
 if-section: 
 if-group elif-groupsopt else-groupopt endif-line 
 if-group: 
 # if constant-expression new-line groupopt 
 # ifdef identifier new-line groupopt 
 # ifndef identifier new-line groupopt 
 elif-groups: 
 elif-group 
 elif-groups elif-group 
 elif-group: 
 # elif constant-expression new-line groupopt 
 else-group: 
 # else new-line groupopt 
 endif-line: 
 # endif new-line 
 control-line:

For example directives #ifdef and #ifndef check whether the identifier used in the directive is or is not defined as a macro name. Based on this the compiler can either include or skip the code followed the directives in the compilation unit.

For example let assume that some code snippet in a C program allocates memory.
For debug version of the program you could check whether the memory was indeed allocated. In this case you could write

 
 #include <stdlib.h> 
 #include <assert.h> 
  
 int main(void)  
 { 
  
 	char *p = NULL; 
  
 	// ... some code that has to allocate a memory pointed to by p 
  
 	assert( p ); 
 }

If the directive #define NDEBUG is not present and p will be equal to NULL then the program will abend because it would not pass the statement with assert.

However if you will include the directive as it is done in the example above and p will be equal to NULL neverthe less the assert will be ignored.

 
 #include <stdlib.h> 
  
 #define NDEBUG 
  
 #include <assert.h> 
  
 int main(void)  
 { 
  
 	char *p = NULL; 
  
 	// ... some code that has to allocate a memory pointed to by p 
  
 	assert( p ); 
 }

The assert macro uses the directive

#ifdef NDEBUG

inside its own definition something as

 
 #ifdef  NDEBUG 
  
 #define assert(_Expression)     ((void)0) 
  
 #else 
 // definition of assert that issues an error

and behaves accordingly.

You can find more information about these directives in books on C/C++ or in internet.