where do these classes come from?

Hi,

I’m new to wp, I am trying to develop my own theme, which can be seen here: https://www.afzdesign.com/conservatorio_02, I think it’s all ok, in case it’s not please let me know.
Anyhow, the problem I have is trying to have drop-down menus, I found this interesting tutorial, I put the code into the style.css, but the subpages stay there with bulletted lists, when I look at the source I see this code
<li class="page_item page-item-63">
with the number changing depending on the page … where does that come from, and what is generating it?
Any idea how to make it work?

Thank you.