I know that define is a search and replace thing but I dont get what define is doing when the code is like this

Code:
#ifndef DATE_H
#define DATE_H

#include <iostream>
#include <string>

using namespace std;

class date
  {
    private:
      enum m_e_month                    //member enumeration named month
      {
        January=1,
        Febuary=2,
        March=3,
        April=4,
        May=5,
        June=6,
        July=7,
        August=8,
        September=9,
        October=10,
        November=11,
        December=12
      };
      m_e_month m_m_e_month_month;                    //its a member variable m_ of type (m_e_month) named month
      unsigned short int m_sh_I_year;
      unsigned short int m_sh_I_day;
      void makeyear()
        {
          cout<<"enter the year:";
          int a;
          cin>>a;

        }
     blah{}int float month m_m_e_month_mohnth {} ; makgeblah((boool asdfhpsjdj609/^#g) hfljgfc...
  };

#endif
what exactly is "#define DATE_H" defining it certainly isn't making any macros or replacing anything so what does it do and what is it's purpose when there is no replacement?
(When I mean replacement I mean the "1234" in
#define abcd 1234
)