UAC is a feature that was added in Windows Vista and improved in Windows 7. In Vista Warnings occurred all the time when changes were going to made to programs or the operating system. In Windows 7 the UAC recognizes some changes as ok.
The object of UAC is to keep malware (viruses, spyware, etc) and other bad programs from being able top just make changes to your system. When changes are going to occur to the system or programs Windows pops up a UAC box and asks if you want to let what ever program make changes to it. The problem is that most people have no idea what they should let occur or not.
To most people these warnings annoy them and they are not sure how to answer. If you are installing or updating programs when it pops up then answer that the changes are ok. If you were not doing updates or installs, then look at the program name and maker and if you know approve. If not disapprove.
To make these messages not appear or to appear less often you can go to Control Panel and then User Accounts and then Change User Account Control Settings. There will be a slider with four settings you can choose from. The bottom is Never Notify which is strongly suggested not to use. Yje next up is Notify Me Only When programs try to Change My Computer and not dim desktop. This is one I use as I get warnings when programs try changes but not dim desktop and limit me til I answer it.
The next one up is the Default which answers same but dims desktop until you answer. The top one is Always Notify and you will get lots of UAC popups but if you install lots of stuff from web you may want to use fro security knowing what is being installed.
Thanks Glenda for question.