Normalisation can mean different things depending on the context, but can mostly be summarized by saying that, when undertaking comparisons, it is necessary to give the compared elements an even footing. More specifically the elements being compared often different between each other in terms of size, and perhaps in other respects too. The elements may be types that already have different abundances. The idea is that some aspects in the nature of their differences will cause different responses in the system and this will weaken, not to say invalidate, response comparison.
In Next Generation Sequencing terms, normalisation is a crucial stage in RNA-Seq where abundance is a principal part of the analysis.