Save Post Meta

<?php

function wedevs_save_top( $post_id ) {
    // make sure something has been submitted from our nonce
    if ( !isset($_POST[$key . '_wpnonce']) )
        return $post_id;

    // verify this came from the our screen and with proper authorization,
    // because save_post can be triggered at other times
    if ( !wp_verify_nonce($_POST[$key . '_wpnonce'], basename(__FILE__)) )
        return $post_id;

    // verify if this is an auto save routine.
    // if it is our form and it has not been submitted, dont want to do anything
    if ( defined('DOING_AUTOSAVE') && DOING_AUTOSAVE )
        return $post_id;

    // lastly check to make sure this user has permissions to save post fields
    if ( !current_user_can('edit_post', $post_id) )
        return $post_id;

    if( isset( $_POST['top_ad'] ) ) {
        update_post_meta( $post_id, 'top_ad', $_POST['top_ad'] );
    }
}

add_action( 'save_post', 'wedevs_save_top' );