Arduino web server control with HTML

my english is bad, so sorry for that…I am completely newbie with Arduino and I need some help and explanation. I work on some project that can control some led with arduino web server for school . For control Arduino pins from web page, u use zoomkats code for links and button. Also, almost every web server control examples are made using link ( a href) .Of course, everything work fine, and i can control some led, but i get problem with my teacher. He ask me many question , like : how this stuff with link actually work and why.?( communication protocol, what this a href actually do. Why it prints in Serial monitor GET. How can it prints GET but it’s not used HTML form for GET in code, and so on…? I really don’t know the answer. Here is just part with a href and readString part for this a href values

client.println(“ON”);
client.println(“OFF”);




if(readString.indexOf(‘2’) >0)//checks for 2
{
digitalWrite(5, HIGH); // set pin 5 high
Serail.println(“Led 5 On”);
}
if(readString.indexOf(‘3’) >0)//checks for 3
{

digitalWrite(5, LOW); // set pin 5 low
Serial.println(“Led 5 Off”);
}