Stacked = More than one sound played to create one unique sound. EXAMPLE-> You take 2 or more kick drum samples and play them at the same time so they sound like one phat kick. You find 2 you like and set them to compliment each other to create the sound you want. Either resample (and process maybe) em or just play them at the same time.
Processed = Using E.Q.ing, Compression, EFX (chorus, reverb,etc) on sounds (samples) to make the sound how you want them (loud, crisp, phat,...). AKA - TWEAKING SAMPLES.