How does "Custom HTML header" works?

Hi,

I am currently working around Cells to check if and how I can had a message when users gon on to remind them some good practices.
I came accross a paramater that may fill my need : “Custom HTML header” but nothing I tried seemed to have an effect whatsoever, and I couldn’t find a description on how it works or what it’s supposed to do on the website.

I would greatly appreciate if someone can help me

Regards,
A. Desombre