Arduino Sensor Experiment | Anjelica Harlow + Albert Lui

Hi there. This is Albert and Angelica, we are now officially a group. I'm leaving for Toronto soon and wont be in class on next week so I figured I would get this out of the way as soon as possible. Pretty simple, I'm using a momentary push button as my 'sensor.'
Whenever the button is pressed down, the red LED lights up. I actually had to build my circuit with a resistor. Our textbook never really tells us why this is necessary and how the circuit actually works. I understand the code completely, but I don't quite understand the electronics. I have a few questions I'm hoping to ask Dominic in person.

Pressing the momentary button down to light up the red LED.

/*
Turns on an LED when the momentary switch is held down.
 */

int led = 13;
int button = 7;
int val = 0;

void setup() {              
  // initialize then digital pins as input/output for the LED an switch
  pinMode(led, OUTPUT);
  pinMode(button, INPUT);
}


void loop() {
  val = digitalRead(button);
  if (val == HIGH) { //switch is pressed
    digitalWrite(led, HIGH);   // turn the LED on (HIGH is the voltage level)
  } else {
    digitalWrite(led, LOW);    // turn the LED off by making the voltage LOW
  }
}