HiFi is a head injection and foot injection plugin (or head include, foot include, head insert, foot insert, what ever you want to call it). It basically allows you to inject <script>, <style>, <meta> and any other code you want into the head and foot areas of your posts and pages. The code injected is page-specific, this means that only the pages you want code inserted into will be affected.
This plugin is most useful when you have specific functionality that needs to be added on a per-post/page basis.