sub = sub-exposure, one of the many that will make up your total exposure.
Don't be afraid to ask - there are no stupid questions, only stupid answers
I don't want to sound like a stuck record, but as I said in your DSLR thread, exposing the image correctly is absolutely key to getting good results. The best thing to do is to expose for long enough that the background sky level (the big peak in the histogram in most images) has higher values than the read noise (+thermal noise).