What is frame, How OnGuI is called every frame?

Hi, I'm a newbie to the unity environment, trying to understand unity scripting API with .netFramework. I came across a method known as OnGUI which by default gets called every frame. Now i Don't understand the concept of Frame,I need to understand the mechanism behind the calls to this function.Please some one explain that anything written inside this function is rendered once or repeatedly as the function's nature is iterative.