This plugin turns off featured image display on all pages and “single” post pages. Use it when you have a theme that displays featured images in a way you don’t like (for instance, the Twenty Something themes all display the images large above the content, which is rarely what I want).
If the featured images are used elsewhere in the theme — in a posts grid, for instance — that will not be affected. Facebook and other systems who use your page’s metadata to locate the featured image, should also continue to work.
This is a good plugin to use in the case where you don’t have much existing content, or if you switched to a theme that displays the featured image in a way you don’t like (which for me is “at all”) on all your old posts, and you can’t turn it off through a theme setting.
If you’re happy with the way featured images display on your site, with just the occasional exception, this is the wrong plugin for you. In that case I’d recommend the plugin Conditionally display featured image on singular pages and posts, which lets you disable featured images on specific pages rather than across the board.