Whenever you make changes to theme files, any php code changes or when you install an extension to your opencart website you will likely need to clear the cache to see those changes reflected in the site. Opencart has ocmod and sometimes vqmod which is a way of customizing opencart files without touching core files. This system caches the resulting changed files into system/storage/modification for ocmod and vqmod/vqcache for vqmod. Those systems cache the resulting file changes so Opencart can run faster. This tutorial shows you how to clear the cache in Opencart 2.3+. This does not apply to any page cache systems and if you need to clear a page cache in your opencart you likely have a third party caching plugin installed and you need to find it in the admin and clear that cache too.