Tic Tac Toe Explanation Needed

Can someone explain me the parts in BOLD letters. It will really help. I wanna know how the game is done. and also the function of posswin(). Thanks

Code:

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#include<stdio.h>
#include<conio.h>
#include<stdlib.h>
int board[10] = {2,2,2,2,2,2,2,2,2,2};
int turn = 1,flag = 0;
int player,comp;
void menu();
void go(int n);
void start_game();
void check_draw();
void draw_board();
void player_first();
void put_X_O(char ch,int pos);

main()
{
 clrscr();
 _setcursortype(_NOCURSOR);
 menu();
 getch();
 return(0);
}
void menu()
{
 int choice;
 printf("\nWelcome To The Tic Tac Toe Game");
 printf("\n--------MENU--------");
 printf("\n1 : Play with X");
 printf("\n2 : Play with O");
 printf("\n3 : Exit");
 printf("\nEnter your choice:>");
 scanf("%d",&choice);

 turn = 1;
 switch (choice)
 {
  case 1:
  player = 1;
  comp = 0;
  player_first();
  break;
  case 2:
  player = 0;
  comp = 1;
  start_game();
  break;
  case 3:
  exit(0);
  default:
  menu();
 }
}
int make2()
{
 if(board[5] == 2)
  return 5;
 if(board[2] == 2)
  return 2;
 if(board[4] == 2)
  return 4;
 if(board[6] == 2)
  return 6;
 if(board[8] == 2)
  return 8;
 return 0;
}
int make4()
{
 if(board[1] == 2)
  return 1;
 if(board[3] == 2)
  return 3;
 if(board[7] == 2)
  return 7;
 if(board[9] == 2)
  return 9;
 return 0;
}
int posswin(int p)
{
// p==1 then X  p==0  then  O
 int i;
 int check_val,pos;
 if(p == 1)
  check_val = 18;
 else
  check_val = 50;
 i = 1;
 while(i<=9)//row check
 {
  if(board[i] * board[i+1] * board[i+2] == check_val)
  {
  if(board[i] == 2)
    return i;
  if(board[i+1] == 2)
    return i+1;
  if(board[i+2] == 2)
    return i+2;
  }
  i+=3;
 }
 i = 1;
 while(i<=3)//column check
 {
  if(board[i] * board[i+3] * board[i+6] == check_val)
  {
  if(board[i] == 2)
    return i;
  if(board[i+3] == 2)
    return i+3;
  if(board[i+6] == 2)
    return i+6;
  }
  i++;
 }
 if(board[1] * board[5] * board[9] == check_val)
 {
  if(board[1] == 2)
  return 1;
  if(board[5] == 2)
  return 5;
  if(board[9] == 2)
  return 9;
 }
 if(board[3] * board[5] * board[7] == check_val)
 {
  if(board[3] == 2)
  return 3;
  if(board[5] == 2)
  return 5;
  if(board[7] == 2)
  return 7;
 }
 return 0;
}
void go(int n)
{
 if(turn % 2)
  board[n] = 3;
 else
  board[n] = 5;
 turn++;
}
void player_first()
{
 int pos;
 check_draw();
 draw_board();
 gotoxy(30,18);
 printf("Your Turn :> ");
 scanf("%d",&pos);
 if(board[pos] != 2)
  player_first();
 if(pos == posswin(player))
 {
  go(pos);
  draw_board();
  gotoxy(30,20);
  textcolor(128+RED);
  cprintf("Player Wins");
  getch();
  exit(0);
 }
 go(pos);
 draw_board();
 start_game();
}
void start_game()
{
 // p==1 then X  p==0  then  O
 if(posswin(comp))
 {
  go(posswin(comp));
  flag = 1;
 }
 else
 if(posswin(player))
  go(posswin(player));
 else
 if(make2())
  go(make2());
 else
  go(make4());
 draw_board();
 if(flag)
 {
  gotoxy(30,20);
  textcolor(128+RED);
  cprintf("Computer wins");
  getch();
 }
 else
  player_first();
}
void check_draw()
{
 if(turn > 9)
 {
  gotoxy(30,20);
  textcolor(128+RED);
  cprintf("Game Draw");
  getch();
  exit(0);
 }
}
void draw_board()
{
 int j;
 for(j=9;j<17;j++)
 {
  gotoxy(35,j);
  printf("|      |");
 }
 gotoxy(28,11);
 printf("-----------------------");
 gotoxy(28,14);
 printf("-----------------------");
 for(j=1;j<10;j++)
 {
  if(board[j] == 3)
  put_X_O('X',j);
  else
  if(board[j] == 5)
  put_X_O('O',j);
 }
}
void put_X_O(char ch,int pos)
{
 int m;
 int x = 31, y = 10;
 m = pos;
 if(m > 3)
 {
  while(m > 3)
  {
  y += 3;
  m -= 3;
  }
 }
 if(pos % 3 == 0)
  x += 16;
 else
 {
  pos %= 3;
  pos--;
  while(pos)
  {
  x+=8;
  pos--;
  }
 }
 gotoxy(x,y);
 printf("%c",ch);
}

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