This is a combination of "look here now,or just plain "look"." It is used when you are somewhat frustrated with a person, and want to get their attention real quick and advise them they are incorrect, are missing the point, or do not understand, and therefore you will explain or clarify the situation to them all over again.
Lookit! If you wanna go to the rodeo, you have to wear cowboy boots. Plain and simple!
Lookit bird brain! I am not gonna splain the whole thang all over again. Just do it!