Validation 101 questions

Hi – newbie here. I am having a mental block and am hoping someone can explain it to me clearly.

I understand that themes have a CSS sheet and all the other functions sheets. I’ve even comfortably made some changes. So I thought that everything works off of those. Is that right?

Where I get stuck is when I validate the site. I will see errors but don’t know where to find the source to correct. For example. I went to site report card and I have 18 errors relating to google+. So I click on the corrected code and it gives me all in html and says you can paste this in your site…but where? It’s just all html of the entire site. Doesn’t everything work off of the codes in the CSS and all the other files and that is what generates the output.

I then tried to be what I was thought was logical and located the CSS of the 2 plug ins that use google+ (WP Biogaphica and Really Simple Share) and it validates…so then I am more confused.

So is there an html sheet of the site or do I have to use a validator service and then just go through all of the sheets to find it. I don’t get it.

Thanks