A traffic light is red, yellow, or green.

Nothing else.

Store that as a String and a typo like 'rd' compiles fine. Your program starts. And somewhere, silently, it breaks.

An enum makes that mistake impossible.

Defining an enum

Here is a traffic light as an enum:

enum TrafficLight {
  red,
  yellow,
  green,
}

TrafficLight can only ever be red, yellow, or green. Nothing else compiles.

To use a value, write the enum name, a dot, then the value name:

void main() {
  TrafficLight light = TrafficLight.red;
  print(light); // TrafficLight.red
}

Why not a String?

Here is the same thing done with a String:

void main() {
  String light = 'red';
 
  if (light == 'rd') {   // typo: 'rd' instead of 'red'
    print('Stop.');
  }
}

Dart accepts 'rd'. It is a valid string. But the condition never matches. No error. Just broken behavior.

With an enum, the same typo fails at compile time:

void main() {
  TrafficLight light = TrafficLight.reed; // Error: The getter 'reed' isn't defined for the type 'TrafficLight'
}

The compiler caught it. The program never started with the mistake in it.

Enums in if

Compare an enum value with ==:

void main() {
  TrafficLight light = TrafficLight.red;
 
  if (light == TrafficLight.red) {
    print('Stop.');
  } else if (light == TrafficLight.yellow) {
    print('Slow down.');
  } else {
    print('Go.');
  }
}

No strings. No typos. If you mistype TrafficLight.gren, Dart catches it before the program runs.

Enums in switch

switch is a natural fit for enums. One case per value.

void main() {
  TrafficLight light = TrafficLight.green;
 
  switch (light) {
    case TrafficLight.red:
      print('Stop.');
      break;
    case TrafficLight.yellow:
      print('Slow down.');
      break;
    case TrafficLight.green:
      print('Go.');
      break;
  }
}

Every possible value has a case. Nothing can slip through.

Dart 3 added enhanced enums with fields and methods, and smarter switch patterns that work with them. Dart in Depth covers both.

Summary