not sure how else to explain...
this is just an example....

lets just make up that 4 seconds of any sample can loop perfectly at 80bpm on a 2bar loop... or let me clarify 4seconds=80bpms on a 2bar loop.....

lets say you cut a sample that was 5 seconds... it loops okay but not at the bpm you want... because maybe 5sec=70bpms on a 2 bar loop....
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this is where time stretching comes in.... where you stretch or compress a sample to fit a certain bpm.... so you timestretch that 5 sec sample to a 4 second sample....