Plugin creator’s site: https://wordpress.org/plugins/all-meta-tags/
This plugin helps you manage meta tags inserted invisibly in the headers of your site for search engine optimization.
For an example of how it works, this site’s headers include the phrase “All Meta Tags Functionality Check”.
When you look at the source code for any page within the publish.illinois.edu/plugindemo site, this line of code will be included:
<meta name='keywords' content='All Meta Tags Functionality Check' />