Digital audio measures a signal many thousands of times per second. According to the Nyquist theorem, a system can only capture frequencies up to half its sample rate, which is why CD audio uses 44.1 kHz to cover a nominal 20 kHz bandwidth. Lower the rate and the upper frequencies disappear; lower it without adequate filtering and false tones called aliasing fold back into the audible range, giving a brittle, metallic edge.
Classic samplers worked at modest rates: the E-mu SP-1200 sampled at about 26 kHz and the MPC60 at 40 kHz. Downsampling effects recreate that character on purpose; in Apple’s Bitcrusher a 10x setting turns 44.1 kHz into 4.41 kHz without changing pitch or speed. Lo-fi producers use mild downsampling to give drums and keys the softened, slightly grainy feel of vintage hardware.