Basically what this means is that if you have a Gmail account, whenever you do a search for something in regular google search or youtube, it will track and record everything you do if your still logged in. The finest example is YouTube. I've went from YouTube (while in my email) and noticed that Google had me logged in YouTube under my email account. Honestly, that really pisses me off because I did not log into YouTube by choice and do not want them logging me into YouTube without my permission. I basically have to log out of that google account just to log into the YouTube account I originally created before google bought YouTube.
I don't know what you're pissed off about. Your e-mail [including gmail] account and your YouTube account are linked. Therefore, the google system makes it easy for your account to be synced when logged in, as in a universal login system.
When Google bought YouTube, they asked YOU to link your account [whatever account at the time was called upon], and you obliged, you accepted this.
Before Google's acquisition of YouTube, I had my youtube account linked to an outside service. When Google bought YouTube, I made it a point to link the youtube account with the gmail account. This way, there's no confusion.