its when you sample different keys of an instrument in order to build a more authentic representation of it, it allows your to play create a more accurate sounding scale of notes.
for example, if you just sample one note of a piano and pitch that note up or down an entire scale, it will not produce an accurate representation of what those notes should sound like (after the first few semi-tones). but if you take 3 or 4 samples from the piano (A3, C3, F3, G3 for example) and then pitch those 4 samples up and down correspondingly to creat a full scale you will have a more realistic sounding sample set of the instrument.
its best to sample from every at least every third semitone if your after a more realistic sounding sampled instrument.
