How can I add a Description column to the media library browser screen?

I think I’m getting hung up on this line:

echo substr(strrchr($meta['attachment_content'], "https://wordpress.stackexchange.com/" ), 1);

Yep! Try this version of the description_value function:

function description_value($column_name, $id) {
    echo  get_the_content($id);
}

Media items are posts of the type attachment. The description is stored in the post_content property of the post object. You can see all of the fields by doing a print_r( get_post( $id ) ); just in case you wanted to make some other columns. By the way, wp_get_attachment_metadata will give you access to the EXIF data of the image, which I don’t think you want here.