Assisted pullups are done on a machine that uses the weight stack as a counterbalance to your own weight - the less weight you select, the harder the pull up is - so for example if I set the weight at 25kg, I'm pulling my own weight minus 25kg - the idea is that as you get stronger you gradually reduce the weight you use until you're using no counterbalance at all - then and only then do you do pull-ups unaided.