The green line shows how many frames you have loaded into RAM. It moves because you don't have enough ram in your system to load up every frame at the current resolution. There are also holes in the green line when all animation stops in the comp. AE is smart enough to not cache repeated frames.
Thanks for the reply, I have 8gb of DDR3, surely that is suffient for a 10 second 1080p video? I alos maxed the RAM in the settings.
That's simply not how it works. The program itself needs RAm, your operating system needs RAM, other drivers and processes need RAM. even under ideal conditions, there are never 8GB available to AE. That aside, AE will not cache frames in RAM that do not change, e.g. static text just sitting there on a still background. Also there is the disk cache which will hold frames and the program will play them from there as it will footage files if nothing is applied to them. Lastly, whenever effects are involved that can be rendered in realtime, the program will not necessarily cache to RAM or disk... So many things going on and the whole matter is much more complicated. Just because you have sufficient RAM doesn't mean at al lthat AE would cram it full with data when it doesn't need to...
There are two things to do:
check out Adobe's recommended memory and processing settings. THEY'RE HERE.
Next, don't make the mistake of thinking you can learn AE intuitively, just by fooling around with it. Take advantage of the valuable info for beginners found at this URL:
AE has an extremely nasty habit of hosing the newcomer who thinks, "hey, how touch can this stuff be?"