You can write analog values, too -- but only on certain pins. Use digital 9, 10, and 11. (That seems confusing, I know.)
Try moving your LED lead from pin 13 to pin 9. Keep everything else the same.
Now open a new sketch: File->Examples->03.Analog->AnalogInOutSerial
Now what happens when you twist the dial?
In the AnalogInOutSerial sketch, you'll see a line like this:
So what is analogOutPin? If you look earlier in the sketch, you'll see:
const int analogOutPin = 9;
But you'll also see these lines:
// read the analog in value: sensorValue = analogRead(analogInPin); // map it to the range of the analog out: outputValue = map(sensorValue, 0, 1023, 0, 255);
Those are because analogRead gives you values between 0 and 1023,
but analogWrite can only take values between 0 and 255.
So map takes the values you read, from 0 to 1023,
and shrinks them down to make sure they aren't too big to write.
When you're writing a program, it can be really useful to print out the values you're reading, in case they're not what you expect.
To print something out, first do this inside setup():
Then you can print things out inside loop() like this:
Serial.print("sensor = "); Serial.println(sensorValue);
println means print something and end the line.