map helper method ES6

var numbers = [1, 4, 9];
var roots = numbers.map(Math.sqrt);

In the above code snippets everything is working fine ,but unable to visualize the mechanism of map helper method syntax using above problem. How the above code is working under the hood ,that is the confusion? https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map