I used Profile builder plugin in my WordPress site.The plugin has not functionality auto log in after Email confirmation. I’ve found this on the plugin developers website. But i don’t know how to use this code snippet. Please explain this for me.

After users confirm their emails this function will make them auto login in their accounts.

wppb_activate_user is located in Profile Builder: /features/email-confirmation/email-confirmation.php and /front-end/register.php

init is located in wp-settings.php

/*
 * Auto Login After Email Confirmation. Tags auto login, email confirmation
 */

add_action( 'wppb_activate_user', 'wppb_custom_autologin_redirect', 10, 3 );
function wppb_custom_autologin_redirect( $user_id, $password, $meta ){
    // hack to fix conflict with WP Voting Contest in C:wwwpb20wp-contentpluginswp-voting-contestincludesvotes-save.php lines 420, 421, 422
    // basically WP Voting Contenst will login any email field that's submitted via POST. So we're overwriting the global $current_user for this particular instance.
    global $current_user;
    $current_user = 0;

    $token = wppb_create_onetime_token( 'pb_autologin_'.$user_id, $user_id );

    $location = add_query_arg( array(
        'pb_autologin' => 'true',
        'pb_uid'       => $user_id,
        'pb_token'     => $token,
    ), home_url());

    echo "<script> window.location.replace('$location'); </script>";
}

add_action( 'init', 'wppb_custom_autologin' );
function wppb_custom_autologin(){
    if( isset( $_GET['pb_autologin'] ) && isset( $_GET['pb_uid'] ) &&  isset( $_GET['pb_token'] )  ){
        $uid = $_GET['pb_uid'];
        $token  = $_GET['pb_token'];
        require_once( ABSPATH . 'wp-includes/class-phpass.php');
        $wp_hasher = new PasswordHash(8, TRUE);
        $time = time();

        $hash_meta = get_user_meta( $uid, 'pb_autologin_' . $uid, true);
        $hash_meta_expiration = get_user_meta( $uid, 'pb_autologin_' . $uid . '_expiration', true);

        if ( ! $wp_hasher->CheckPassword($token . $hash_meta_expiration, $hash_meta) || $hash_meta_expiration < $time  ){
            //wp_redirect( $current_page_url . '?wpa_error_token=true' );
            die (' You are not allowed to do that. ');
            exit;
        } else {
            wp_set_auth_cookie( $uid );
            delete_user_meta($uid, 'pb_autologin' . $uid );
            delete_user_meta($uid, 'pb_autologin' . $uid . '_expiration');
            wp_redirect( home_url() . '/edit-profile/' );
            exit;
        }
    }
}

function wppb_create_onetime_token( $action = -1, $user_id = 0 ) {
    $time = time();

    // random salt
    $key = wp_generate_password( 20, false );

    require_once( ABSPATH . 'wp-includes/class-phpass.php');
    $wp_hasher = new PasswordHash(8, TRUE);
    $string = $key . $action . $time;

    // we're sending this to the user
    $token  = wp_hash( $string );
    $expiration = $time + 60*10;
    $expiration_action = $action . '_expiration';

    // we're storing a combination of token and expiration
    $stored_hash = $wp_hasher->HashPassword( $token . $expiration );

    update_user_meta( $user_id, $action , $stored_hash ); // adjust the lifetime of the token. Currently 10 min.
    update_user_meta( $user_id, $expiration_action , $expiration );
    return $token;
}