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.

Hi, Using the context is very important in understanding the complete meaning of a sentence. In simple terms, to normalize is to kind of make sense from a lot of data. Thus it helps to reduce the confusion, in a sense to bring the results to a particular base and to reduce the many extras in the data. In a very loose way to sort of average the entire data to a particular point or a base. Details will have to be read as being noted particular to the experiment. Good luck!