In angular.js, what does an ampersand in the directive mean?

In this code:

app.directive( 'myCustomDirective', function() {
  return {
    restrict: 'EA',
    scope: { value: '=myCustomDirective', clickable: '&', editing: '=' },
    template: '<span ng-click="handleClick()" ng-bind="value"></span>',
    ...

What does clickable: '&' denote?

When I do find Angular documentation, I cannot understand it. It seems to be written in a catch-22 code that can be understood only by people who already understand what is being explained.

For example, I found this video, which apparently explains the & thing, but he may as well have been speaking Mandarin.