In includes/languages/english/YOUR_TEMPLATE/meta_tags.php, you should have definitions for both of those constants:
// page title
define('TITLE', 'Zen Cart!');
// Site Tagline
define('SITE_TAGLINE', 'The Art of E-commerce');
To change the definition, change the text in between the second set of single quotes.
// page title
define('TITLE', 'My Store');
// Site Tagline
define('SITE_TAGLINE', 'hello');
To remove them entirely, remove the text, but leave the single quotes.
// page title
define('TITLE', '');
// Site Tagline
define('SITE_TAGLINE', '');
Never remove the upper case text.