Enqueue Scripts and styles with timestamp versioning in WordPress.

functions.php

<?php 
/* Use this when parent theme used */

wp_register_style('main-css', untrailingslashit( get_template_directory_uri() ).'css/main.css', ['bootstrap'], filemtime(untrailingslashit( get_template_directory() ).'css/main.css'), 'all');
wp_enqueue_style('main-css');

wp_register_script('main', untrailingslashit( get_template_directory_uri() ).'js/main.js', ['jquery', 'slick-slider'], filemtime(untrailingslashit( get_template_directory() ).'js/main.js'), true);
wp_enqueue_script('main');



/* Use this when child theme used */

wp_register_style('main-css', untrailingslashit( get_stylesheet_directory_uri() ).'css/main.css', ['bootstrap'], filemtime(untrailingslashit( get_stylesheet_directory() ).'css/main.css'), 'all');
wp_enqueue_style('main-css');

wp_register_script('main', untrailingslashit( get_stylesheet_directory_uri() ).'js/main.js', ['jquery', 'slick-slider'], filemtime(untrailingslashit( get_stylesheet_directory() ).'js/main.js'), true);
wp_enqueue_script('main');


/* 
<h5>More Information</h5>

The returning path does not contain a trailing slash.
An example output of get_stylesheet_directory() is /home/user/public_html/wp-content/themes/my_theme
In the event a child theme is being used, that is the directory that will be returned, not the parent theme directory (use get_template_directory() instead if you want the parent directory).
To retrieve the URI of the stylesheet directory use get_stylesheet_directory_uri()
To retrieve the path of a parent theme, use get_template_directory()

*/