A glitch is something that happens to your tama that's not supposed to happen or is not normal.
Like say if you reset your tamagotchi and there are 100 balls in the food lists.
Or if you get poop in your shop and feed to your tamagotchi and your tamagotchi throws up.
Those are some examples of glitches. If you're lucky, none of your tamas will ever have to experience a glitch.
GotchiGirl96