Directives are what makes Angular so magical in my opinion. At a high level, directives are markers on a DOM element (such as an attribute, element name, comment or CSS class) that tell AngularJS's HTML compiler ($compile) to attach a specified behavior to that DOM element (e.g. via event listeners), or even to transform the DOM element and its children.
Some popular built-in directives include, ng-model, ng-repeat, ng-if, ng-show, ng-hide, and ng-class.
You can also opt to write your own custom directives!
Good rule of thumb for when to use directives is when you need to modify the DOM, especially with jQuery.
You can also treat directives as UI-kits that can serve as a template.
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