This plugin fixes a validation problem related with RFC 4685 atom feed extension: some validators (e.g., w3c) don’t like the thr:total extension. Therefore, this plugin fixes this problem removing the tag from the atom feed without any hack to wordpress code.
Should I modify any code in wordpress?
Not needed. You have to just upload the files
If I de-activate this plugin will it affect my blog?
No. This plugin uses some of function of wordpress. WordPress has nothing to with plugin. So de-activating this plugin is 100% safe.