Use of arrow keys to generate movement in game

I wanted to know how the developer of the Snake game in C ++ merged with C made the snake move from the keyboard arrows.

I think the part it moves on is the time that compares tecla=='K' tecla=='H' and so on, but I wanted to know why it used those letters, and the main one.

Why does not the snake move when you press the arrow that happens to be the letters?

Here's the code

  #include<windows.h>
  #include<stdio.h>
  #include<conio.h>
  #include<iostream>
  #include <stdlib.h>
  #include <time.h>


  //Código feito com base em tutoriais na internet, desenvolvido por  
  Gustavo    Ferreira do 2 semestre do Curso de Jogos Digitais da FATEC   
  SAO CAETANO DO SUL.

  void mgotoxy(int x, int y)
  {
  SetConsoleCursorPosition(GetStdHandle(STD_OUTPUT_HANDLE),(COORD)          
  {x,y});//Percorre a matriz    
  }

  main() 
  {
  int x,d=2,cx[300]={1,2},cy[300]={7,7},t=1,mx,my,velo=100,velo2=5;
  char niv;

  char tecla='a';
  int opcao;
  int pontos=0;
  int nivel = 1;



  for(x=0;x<18;x++)
  { mgotoxy(0,x); //vertical esquerda.//

  printf("%c",219);
  }
  for(x=0;x<50;x++)
  { mgotoxy(x,0); //horizontal superior//
  printf("%c",219);
  }
  for(x=0;x<18;x++)
  { mgotoxy(50,x); //vertical direita//
   printf("%c",219);
  }
  for(x=0;x<51;x++)
  { mgotoxy(x,18); //horizontal inferior.//
  printf("%c",219);
   }

  srand(time(NULL));
  mx=(rand()%49)+1;
  my=(rand()%17)+1;

  velo = 200;

  while(tecla!='s')
  { while(tecla!='s'&&!(tecla=kbhit()))

  { for(x=t;x>0;x--)
  { cx[x]=cx[x-1];
  cy[x]=cy[x-1];
   }

  if(d==0)cx[0]--;
  if(d==1)cy[0]--;
  if(d==2)cx[0]++;
  if(d==3)cy[0]++;
  mgotoxy(cx[t],cy[t]);
  printf(" ");
  if(mx==cx[0]&&my==cy[0])
  { t++;
  pontos++;
  mx=(rand()%25)+1;
  my=(rand()%17)+1;
  velo-=5;
  velo2+=5;

    }
   mgotoxy(cx[0],cy[0]);

   printf("%c",219);

   mgotoxy(mx,my);
   printf("%c",1);
   mgotoxy(55,10);

   printf ("Pontos: %d",pontos);
   mgotoxy(55,5);
   printf ("Nivel: %d",nivel);
   mgotoxy(55,3);
   printf ("Velocidade: %d",velo2);
   mgotoxy(3,22);

   printf ("Jogo desenvolvido por Gustavo Ferreira");
   Sleep(velo);
   for(x=1;x<t;x++)
   { if(cx[0]==cx[x]&&cy[0]==cy[x])tecla='s';
   }
   if(cy[0]==0||cy[0]==18||cx[0]==0||cx[0]==50)tecla='s';

     }
   if(tecla!='s')tecla=getch();
   printf("%c",tecla);
   if(tecla=='K')d=0;
   if(tecla=='H')d=1;
   if(tecla=='M')d=2;
   if(tecla=='P')d=3;
   if(cy[0]==0||cy[0]==18||cx[0]==0||cx[0]==26)tecla='s';

  }
 system("cls");
 system("pause");

 printf ("\n\n\tVOCE PERDEU\n\n");

 printf ("\n\n\tVOCE FEZ %d PONTOS",pontos);







   getch();
    }