4 7-segment display

Can someone help me understand the code

E-GIZMO 4 7-SEGMENT DISPLAY

Please Refer to this manual

MANUAL

It is hard for me to understand this because im still learning

Please post the code here to avoid everyone having to follow a possibly dangerous link to it.

/*
    4 Digit Display v 2.0

 This sample program functions 
 as a 4 digit 7 segment display 
 counter.

 Codes by:
 e-Gizmo Mechatronix Central
 http://www.e-gizmo.com

 */
// Data, Strobe, and clock LD1207
const int STB = 11;
const int CLK = 12;
const int DIN = 13;

char Num[10] = {0xBF, 0x06, 0xDB, 0x4F, 0xE6, 0x6D, 0xFD, 0x07, 0xFF, 0x67};    //0 

byte Digit0;
byte Digit1;
byte Digit2;
byte Digit3;

void setup() {
  pinMode(STB, OUTPUT);
  pinMode(CLK, OUTPUT);
  pinMode(DIN, OUTPUT);
  
  digitalWrite(STB, HIGH);
  digitalWrite(CLK, HIGH);
  digitalWrite(DIN, HIGH);
  
  SET_DISP_MODE();
  SET_DATA_MODE();
  DISP_OFF();
  DISP_ON();
  SET_DISP_CTRL(4 | 0x08);
}  

void loop() {
  SEND_DATA(0x00, Digit0, HIGH);
  SEND_DATA(0x02, Digit1, LOW);
  SEND_DATA(0x04, Digit2, HIGH);
  SEND_DATA(0x06, Digit3, LOW);
  delay(1000);
  Digit0++;
  Digit1++;
  Digit2++;
  Digit3++;
  SEND_DATA(0x00, Digit0, LOW);
  SEND_DATA(0x02, Digit1, HIGH);
  SEND_DATA(0x04, Digit2, LOW);
  SEND_DATA(0x06, Digit3, HIGH);
  delay(1000);
  Digit0++;
  Digit1++;
  Digit2++;
  Digit3++;
  if(Digit0 == 10) {
    Digit0 = 0;
    Digit1 = 0;
    Digit2 = 0;
    Digit3 = 0;
  }    
}

void SET_DISP_MODE(void) {
  digitalWrite(STB, LOW);  
//  WRITE_COMMAND(0x00);    //4 Grids, 12 Segments
//  WRITE_COMMAND(0x01);    //5 Grids, 11 Segments
//  WRITE_COMMAND(0x02);    //6 Grids, 10 Segments
  WRITE_BYTE(0x03);    //7 Grids, 9 Segments    (default)
  digitalWrite(STB, HIGH);  
}  

void SET_DATA_MODE(void) {
  digitalWrite(STB, LOW);  
  WRITE_BYTE(0x40);    //Increment Address after Data has been Written    (default)
//  WRITE_COMMAND(0x44);    //Fixes Address
  digitalWrite(STB, HIGH);  
}

void SEND_DATA(byte addr, byte data, boolean LED) {
  digitalWrite(STB, LOW);  
  WRITE_BYTE(0xC0 | addr);    //addr = 00H to 0DH
  WRITE_BYTE(Num[data]);
  WRITE_BIT(LED);
  digitalWrite(STB, HIGH);  
}

void SET_DISP_CTRL(byte inten) {
  digitalWrite(STB, LOW);  
  WRITE_BYTE(0x80 | inten);    //inten = 0 to 7
  digitalWrite(STB, HIGH);  
}  

void DISP_ON(void) {
  digitalWrite(STB, LOW);  
  WRITE_BYTE(0x80 | 0x08);
  digitalWrite(STB, HIGH);  
}  

void DISP_OFF(void) {
  digitalWrite(STB, LOW);  
  WRITE_BYTE(0x80 & ~0x08);
  digitalWrite(STB, HIGH);  
}  

void WRITE_BYTE(byte db_data)
{
      digitalWrite(DIN, db_data & 1);
      digitalWrite(CLK, LOW);  
      digitalWrite(CLK, HIGH);  
      digitalWrite(DIN, db_data & 2);
      digitalWrite(CLK, LOW);  
      digitalWrite(CLK, HIGH);  
      digitalWrite(DIN, db_data & 4);
      digitalWrite(CLK, LOW);  
      digitalWrite(CLK, HIGH);  
      digitalWrite(DIN, db_data & 8);
      digitalWrite(CLK, LOW);  
      digitalWrite(CLK, HIGH);  
      digitalWrite(DIN, db_data & 16);
      digitalWrite(CLK, LOW);  
      digitalWrite(CLK, HIGH);  
      digitalWrite(DIN, db_data & 32);
      digitalWrite(CLK, LOW);  
      digitalWrite(CLK, HIGH);  
      digitalWrite(DIN, db_data & 64);
      digitalWrite(CLK, LOW);  
      digitalWrite(CLK, HIGH);  
      digitalWrite(DIN, db_data & 128);
      digitalWrite(CLK, LOW);  
      digitalWrite(CLK, HIGH);  
}

void WRITE_BIT(boolean db_bit)
{
      digitalWrite(DIN, db_bit);  
      digitalWrite(CLK, LOW);  
      digitalWrite(CLK, HIGH);  
      digitalWrite(CLK, LOW);  
      digitalWrite(CLK, HIGH);  
      digitalWrite(CLK, LOW);  
      digitalWrite(CLK, HIGH);  
      digitalWrite(CLK, LOW);  
      digitalWrite(CLK, HIGH);  
      digitalWrite(CLK, LOW);  
      digitalWrite(CLK, HIGH);  
      digitalWrite(CLK, LOW);  
      digitalWrite(CLK, HIGH);  
      digitalWrite(CLK, LOW);  
      digitalWrite(CLK, HIGH);  
      digitalWrite(CLK, LOW);  
      digitalWrite(CLK, HIGH);  
}