Ah, I see! Apologies for the misunderstanding.
Well these are basically fields that allow you to add text (HTML, including images etc) before and after the display of event pages. It’s basically up to you what you might put in there – some people might like to add a phone number for a ticket hotline, others a disclaimer, others still some extra markup to help event pages gel nicely with their theme – however it is probably not an ideal location for custom CSS and we’d recommend the steps described in the Themer’s Guide for that:
https://tri.be/support/documentation/events-calendar-themers-guide/