Rest and Spread

Rest and spread are two new operators in the new ES6 (ECMAScript 6) standard which Javascript implements.  Javascript development in Drupal 8 Core is now done in ES6.  See: https://www.drupal.org/node/2815083

Both Rest and Spread are represented with 3 dots (…).

REST (“the rest of the parameters”) is used in function calls where you want to treat an indefinite number of arguments like an array, like this:

/* 
 * accept any number of strings 
 */
function concat_words(prefix, ...words) {
  return prefix + words.join(" ");
}
concat_words("The sentence is: ", "I", "can", "use", "any", "number", "of", "arguments");
//  => "The sentence is: I can use any number of arguments"

The rest operator takes however many arguments are “left” at the end of a list of arguments, and puts them in an array. 

Examples: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/rest_parameters#Examples

SPREAD is the opposite, and is used where you have an array but want to have  a list of elements.

var arr1 = [1, 2, 3];
var arr2 = [4, 5, 6];
// instead of using concat to join two arrays
both = arr1.concat(arr2);
// use this which is easier to visualize
both = [...arr1, ...arr2];

Examples: https://davidwalsh.name/spread-operator

Where does ES6 work?  Pretty much any modern browser.  Here is a browser compatibility guide.

You can use a transpiler like babel (https://babeljs.io) to generate Javascript for older browsers.