Okay kiddo, imagine you have a toy box with lots of toys inside. Sometimes you want to play with a certain toy, but you have to dig through all the other toys to find it. This is kind of like what happens with the internet.
You know how sometimes you use the internet to watch videos or play games on your tablet? Well, lots of other people around the world use the internet too, and sometimes they all want to do things at the same time. For example, there might be a popular game that lots of people want to play, or maybe a new TV show episode is released and everyone wants to watch it at the same time.
This is when the "internet rush hour" happens. Just like cars on the road during rush hour, there are lots of people using the internet at the same time, and things can get really slow and crowded. It's kind of like trying to find your favorite toy in the toy box when all the toys are mixed up and in the way.
But don't worry - lots of really smart people work hard to make the internet faster and better all the time, so it doesn't always have to be slow!