PIC18F4550 + 16x2 LCD code query - microcontroller

Question

I'm a beginner in MCU programming and also very eager to learn lots about it. Recently I have acquired the "Aptinex development board rev.6" which has many features already attached. I also bought a PIC18F4550 to plug it in the board. I've managed to understand how the basics of I/O operations work, but now I have stumbled on the LCD, which is attached to the development board. I found the code bellow in one of many tutorials pages all over google, and decided to try it out because it seemed simple enough. The code works without problems.
//File name 'LCD_Display_String_Original.c'
#define _XTAL_FREQ 8000000
#define RS LATD0
#define EN LATD1
#define D4 LATD2
#define D5 LATD3
#define D6 LATD4
#define D7 LATD5
#include <xc.h>
#include "lcd.h"
void delay_ms(unsigned int delay_value)
{
while(delay_value-- > 0)
// While desired delay value is more than 0, decrement that value by 1 after each 10ms
{
__delay_ms(1);
// Wait 10ms
}
}
int main()
{
TRISD = 0b00000000;
Lcd_Init();
while(1)
{
Lcd_Clear();
Lcd_Set_Cursor(1,1);
Lcd_Write_String("Hello world!");
delay_ms(1000);
}
}
The question that I have is, could someone kindly please explain what is going on in the function Lcd_port (char a);, located in the lcd.h file bellow. Can't understand what conditions are tested and how they are true or false.
//File name 'lcd.h'
#include <xc.h>
#include <stdio.h>
void Lcd_Port(char a)
{
if(a & 1)
D4 = 1;
else
D4 = 0;
if(a & 2)
D5 = 1;
else
D5 = 0;
if(a & 4)
D6 = 1;
else
D6 = 0;
if(a & 8)
D7 = 1;
else
D7 = 0;
}
void Lcd_Cmd(char a)
{
RS = 0; // => RS = 0
Lcd_Port(a);
EN = 1; // => E = 1
__delay_ms(4);
EN = 0; // => E = 0
}
Lcd_Clear()
{
Lcd_Cmd(0);
Lcd_Cmd(1);
}
void Lcd_Set_Cursor(char a, char b)
{
char temp,z,y;
if(a == 1)
{
temp = 0x80 + b - 1;
z = temp>>4;
y = temp & 0x0F;
Lcd_Cmd(z);
Lcd_Cmd(y);
}
else if(a == 2)
{
temp = 0xC0 + b - 1;
z = temp>>4;
y = temp & 0x0F;
Lcd_Cmd(z);
Lcd_Cmd(y);
}
}
void Lcd_Init()
{
Lcd_Port(0x00);
__delay_ms(20);
Lcd_Cmd(0x03);
__delay_ms(5);
Lcd_Cmd(0x03);
__delay_ms(11);
Lcd_Cmd(0x03);
Lcd_Cmd(0x02);
Lcd_Cmd(0x02);
Lcd_Cmd(0x08);
Lcd_Cmd(0x00);
Lcd_Cmd(0x0C);
Lcd_Cmd(0x00);
Lcd_Cmd(0x06);
}
void Lcd_Write_Char(char a)
{
char temp,y;
temp = a&0x0F;
y = a&0xF0;
RS = 1; // => RS = 1
Lcd_Port(y>>4); //Data transfer
EN = 1;
__delay_us(40);
EN = 0;
Lcd_Port(temp);
EN = 1;
__delay_us(40);
EN = 0;
}
void Lcd_Write_String(char *a)
{
int i;
for(i=0;a[i]!='\0';i++)
Lcd_Write_Char(a[i]);
}
void Lcd_Shift_Right()
{
Lcd_Cmd(0x01);
Lcd_Cmd(0x0C);
}
void Lcd_Shift_Left()
{
Lcd_Cmd(0x01);
Lcd_Cmd(0x08);
}
As mentioned before, the code works perfectly fine. I just really need someone to explain how that part works. Any help will be highly appreciated!