I think that this sounds like a biological experiment. I'm not a biologist, but:
I think that a 'negative control' is a control experiment where the response being tested for does not occur.
Normalising is when you offset or scale your results to some baseline. So my guess is that they did the control experiment, measured the results, did the actual experiment, then subtracted (or scaled) the results from the control from the results of the experiment.
It is technical terminology. I would suggest that you should research and understand the meaning of the terms, rather than getting someone to rephrase it in non-technical terms, which might loose some of the meaning.