replaceAll() is a string method. It is used to replace all occurrences of the specified substring inside a string with another substring.

Syntax:
actualString.replaceAll( oldSubstring, newSubstring );

Lets understand with an example :

void main() {
  String sampleString = "I develop, I test.";
  print(sampleString);
  String replacedString = sampleString.replaceAll( "I", "We" );
  print(replacedString);
}

Output:
I develop, I test.
We develop, We test.

Explanation: We have replaced this substring “I” with the new substring “We“.