You won't get the ability to play videos natively or browse youtube and watch videos. This may have worked in 2005, maybe up through 2007 or so, but today I'm betting the site won't even load, and it it does, flash might not work, and if it does, flash 7 certainly won't.
Though, downloading classilla and flash 7 would be my first step for a just-for-grins attempt at making it go like it did ten years ago.
The computational needs of viewing YouTube have long outstripped the capabilities of even machines that were new when it launched.
As a work-around to get some kind of content to view on the machine:
Downloading them with something like 4k Video Downloader (which I use and recommend if you want high quality youtube videos but you have a slow network connection) and then converting them is one option, I believe that there was another tool that could download and convert directly on some old Macs, but the name escapes me at the moment and there's a possibility it has stopped working anyway.