The ae is basically an exposure adjustment, the camera exposes a picture based on an evaluative metering from the overall light that it gets from the whole scene.
Say for example you are shooting a sunset and the light in this situation varies quite a lot from the bright sky around the sun to the dark foreground below and in front, in such a situation the camera does not really know whats the correct exposure, but as a photograper you know what you want.
So either use the right side dial to compensate for what the camera has decided for you, basically it means, you tell the camera what it calculated is not right and you need some adjustment to it by either letting in more light or less light.
You can also do this by pointing on the screen to instruct the camera to calculate the exposure where want to.
If you are not much into photography, have a read about the exposure triangle and will give you good basics