Competitive programming template (C++)

template.cpp

#include <iostream>
#include <cmath>
#include <algorithm>
#include <limits>
#include <vector>
#include <bitset>
#include <cstdlib>
#include <cstdio>
#include <cstring>
#include <time.h>
#include <list>
#include <set>
#include <map>
#include <queue>
#include <cctype>
#include <list>
#include <iterator>
using namespace std;
#define MOD 1000000007LL
#define LL long long
#define ULL unsigned long long
#define LD long double
#define MAX(a,b) ((a)>(b)?(a):(b))
#define MIN(a,b) ((a)<(b)?(a):(b))
#define ABS(x) ((x)<0?-(x):(x))
#define si(n) scanf("%d",&n)
#define sf(n) scanf("%f",&n)
#define sl(n) scanf("%lld",&n)
#define slu(n) scanf("%llu",&n)
#define sd(n) scanf("%lf",&n)
#define ss(n) scanf("%s",n)
#define pnl printf("\n")
#define REP(i,n) for(int i=0;i<(n);i++)
#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define FORR(i,n) for(int i=(n);i>=0;i--)
#define DB(x) cout<<"\n"<<#x<<" = "<<(x)<<"\n";
#define CL(a,b) memset(a,b,sizeof(a))
#define GOLD ((1+sqrt(5))/2)
const double PI=3.14159265358979323846264338327950288419716939937510582097494459230;
void swaps (char *x,char *y){char temp;temp=*x;*x=*y;*y=temp;}
void swapi(int *a,int *b){int temp;temp=*a;*a=*b;*b=temp;}
ULL gcd(ULL a,ULL b){if(a==0)return b;if(b==0)return a;if(a==1||b==1)return 1;
  if(a==b)return a;if(a>b)return gcd(b,a%b);else return gcd(a,b%a);}
#define SIZE 1000001
void preprocess()
{
}//end prepreprocess
void refresh()
{
}//end refresh
void compute()
{
}//end compute
int main()
{
    #ifdef debug
  freopen("input.txt","r",stdin);freopen("output.txt","w",stdout);
  freopen("log.txt", "w", stderr);
    #endif
  int t,i,j;preprocess();
  scanf("%d",&t);
  while(t--)
    {
      compute();
    }//end while
    #ifdef debug
  fprintf(stdout,"\nTIME: %.3lf sec\n", (double)clock()/(CLOCKS_PER_SEC));
    #endif
  return 0;
}

Can you explain the use of files

   #ifdef debug
        fprintf(stdout,"\nTIME: %.3lf sec\n", (double)clock()/(CLOCKS_PER_SEC));
   #endif

can you please explain this.