Multitracks are the most granular form of a recording — every individual audio file from a session. A drum recording might have 8-12 multitracks: kick in, kick out, snare top, snare bottom, hi-hat, toms, overheads, room mics.
Multitracks give a mixing engineer complete control over every element. They're what you hand to a mixing engineer when you want a professional mix.
The distinction matters when buying or selling: stems are pre-mixed groups (easier to work with, less flexible), while multitracks are raw recordings (maximum flexibility, requires mixing skill).