Okay, imagine you have a toy car that you love to play with. Now, imagine another kid comes along and smashes your toy car and runs away without telling you who they are. That's not very nice, right?
Well, something similar can happen on the internet. Sometimes, bad people can use lots of computers all at once and make them all try to visit a website or a service at the same time. This can be like lots of toy cars all trying to enter a tiny garage at once - it just won't work.
This is called a DDoS attack. It makes the website or service super slow or even stop working altogether. But what if the bad people didn't want to keep doing this for a long time, but just did it once and then ran away? That's kind of like what a hit-and-run DDoS attack is.
The bad people use their computers to attack the website or service really hard, but only for a short amount of time - they don't stay long enough for anyone to catch them. It's like they hit your toy car and then run away before you can even see who did it.
This can be really frustrating for the people who run the website or service because they might not even know who did it or why. It's important to make sure your toy car (or your website) is safe, so sometimes people might use special tools to protect themselves from DDoS attacks, including hit-and-run ones.