Reset button

A reset button or reset switch is a part of an electronic machine that makes it start again. It turns the machine off and turns it back on, very fast.
Real reset buttons are on machines, but "reset button" is also a metaphor for other things. It means "thing that makes everything start over" or "thing that puts things back the way they were in the beginning."
In fiction, especially serialized fiction, like television shows, the writers often use a reset button. No matter what happened during the episode, things always go back to the beginning for the next episode.