CHILD_THEME_URI constant defines the child theme directoryURI. This is the path to the child theme directory if a child theme is active. Else, it refers to the parent theme directory.
The following code show an example of the
CHILD_THEME_URI constant being used to enqueue a stylesheet.
wp_enqueue_style( 'example', trailingslashit( CHILD_THEME_URI ) . 'css/example.css' );