If you want to add a custom meta tag to your Magento store, you need to edit the head.phtml file of your template. The location of the file is inside your Magento Installation directory.
You should replace in the path above with the actual folder for your template.
Then you need to edit the file and add your custom meta tag to it.
Finally, you should flush the Magento cache and reload your website in your browser. You can check the source code of the page (Ctrl + U for most browsers) to make sure your meta tag is included properly.