What is ::ng-deep selector in angular?

What is ::ng-deep selector in angular? 

Component styles normally apply only to the HTML in the component's own template.

Use the ::ng-deep shadow-piercing descendant combinator to force a style down through the child component tree into all the child component views. The ::ng-deep combinator works to any depth of nested components, and it applies to both the view children and content children of the component.    

The following example targets all <h3> elements, from the host element down through this component to all of its child elements in the DOM.

Code:

parent.component.html

Parent content.

<app-child></app-child>

parent.component.css

::ng-deep h3 {
  font-style: italic;
}

child.component.html

<h3>Child title</h3>
Child content.

Result:

Parent content.
Child title
Child content.