It's literallly a term invented by dyels.
Going to the gym 4 times a week will yield greater results than going 3 times. Going 6 times will yield greater result than going 4 times. Going everyday will still result in more gains.
If you've been lifting for less than a year, you absolutely don't need rest days because you're a weak bitch tossing around baby weights. Your body recovers from that within a day.
Rest days come into play when you're highly advanced and toss around 4+ plate deadlifts.

tl;dr YOU'RE NOT GAINING MUSCLE BECAUSE YOU'RE NOT TRAINING OFTEN ENOUGH AND HARD ENOUGH.