Does functions.php apply to every page?

the if(is_front_page()) will check if front page is being viewed, then load your css and meta slider .

Codex documention

<?php

    add_action( 'wp_enqueue_scripts', 'my_theme_enqueue_styles' );
    function my_theme_enqueue_styles() {
        # added
        if(is_front_page())
            wp_enqueue_style( 'parent-style', get_template_directory_uri() . '/style.css' );
    }

    add_action( 'init', 'child_theme_init' );
    function child_theme_init() {
        # added
        if(is_front_page())
        add_action( 'storefront_before_content', 'woa_add_full_slider', 5);
    }

    function woa_add_full_slider() {
        # added
        if(is_front_page()){
        ?>
        <div id="slider">
            <?php echo do_shortcode("[metaslider id=252 percentwidth=100]"); ?>
        </div>
    <?php
        }
    }
    ?>