This either means that the username and password information in your wp-config.php file is incorrect or we can't contact the database server at bdd-db-paris.c1eqofcxfee8.eu-west-3.rds.amazonaws.com. This could mean your host's database server is down.
If you're unsure what these terms mean you should probably contact your host. If you still need help you can always visit the WordPress Support Forums.