What it means to me, is that Microsoft trying to make a computer more secure, included that the OS is the Administrator, and you are just a User. The OS has more priority than you, because the OS knows more than you know (or the average bug who uses a computer). That way, people can not come back and stick Microsoft with the problem, because the User does not know what they are doing (and a lot of people do not.)
Before the OS like XP had you make an account and automatically if you made the account as the Administrator, you could mess up your computer. (and yes, people mess up their computer, forget to do things, then they are a security risk to his/her/alien self).
Now if the OS is the Administrator (and just look at the Accounts on your computer, there are Accounts you never have access to, one is Microsoft can put on software on your computer downloaded, if you do not activate the software (you have not bought it) it stays protected and you have no access to it.
This means that the OS is the protector (because people do not know what they are doing with a lot of people) so you will not be able to hack your own computer and obtain software you have not bought but still may be on your computer when you buy it. I have that with Microsoft Startup of Office or something like that. If I never activate it (pay for it) I never get to use it (although right now, I am thinking of deleting it anyway if I can).
In other words, you are not the top dog on your computer, you are just another User, and the OS is got built-in security to protect its self (but still I have about 232 critical updates with Windows 7 because of security issues with IE, OS or whatever on this computer).
So you get to enjoy headaches figuring this all out, like the User Acccount Control and Microsoft gets to advertise that their OS is more secure for you, while that is probably debatbable since you have around 232 critical updates since Windows 7 has launched and you also have Service Pack 1 to fix the OS after it came out, while still denying from Microsoft that there still are other problems that are not as important that Windows 7 will do because it is not fixed all the way, so Microsoft can still say - you don't know how to run your computer and the OS is ours anyway, so you can not screw me in my business so I will make the OS the Administrator and you just another user, and for a lot of people that is fine, it gives them comfort, while to some, it wants to make them pull out their hair or be done with this OS anyway.
And just remember that it is not "YOUR" computer, it is the manufacturer's computer and Microsoft's computer. You just get to use it while you paid a price for it.