Don't understand function extract in this piece of code

This piece of code is from an example about Widget in a book called professional WordPress Design and Development.

I just wonder what does the call to extract($args)?

function widget($args, $instance)
{
extract($args);
echo $before_widget;
$title = apply_filters(‘widget_title’, $instance[‘title’] );
$name = (empty($instance[‘name’] ) ) ? ‘ ‘ : $instance[‘name’];
$bio = (empty($instance[‘bio’] ) ) ? ‘ ‘ : $instance[‘bio’];

if ( !empty( $title ) )
{
echo $before_title. esc_html($title). $after_title;
}

echo ‘<p>Name: ‘ . esc_html( $name ) . ‘</p>’;
echo ‘<p>Bio: ‘ . esc_html( $bio ) . ‘</p>’;
echo $after_widget;
}

//Tony