Okay kiddo, so you know how when you ride a bike, you go fast and then slow, fast and then slow, right? Well, when we talk about someone with rapid cycling, we mean that their moods are like that too - up and down, up and down - but much faster and more intense than just riding a bike.
See, some people have something called bipolar disorder, which means their moods switch back and forth between being really happy (or "manic") and really sad (or "depressed"). Rapid cycling is when someone with bipolar disorder has four or more of these mood swings in a year. That's a LOT, and it can be really exhausting for the person going through it.
So imagine you're on a rollercoaster that goes way faster than most other coasters, and it keeps going up and down, up and down, without stopping. That's kind of what rapid cycling feels like, and it can be very difficult for someone to manage on their own. That's why it's important for them to talk to a doctor or therapist who can help them find ways to stay as stable as possible and keep their moods from going too wild.