Okay kiddo, so an arrestable offence means that if someone does something really bad, the police can take them to jail. It's like if you broke a rule and got put in time out, but for grownups.
Now, what does really bad mean? Well, it means things like hurting someone on purpose, stealing something valuable, or doing something illegal that could hurt lots of people. These are all things that the government says you shouldn't do, and if you do them, the police can arrest you.
But, not every bad thing is an arrestable offence. Sometimes, if you break a small rule, you might just get a time out or a fine. But if you do something really serious, then the police can take you to jail until you have a trial to decide if you should be punished even more.
In short, an arrestable offence means doing something really wrong that could hurt people, and the police can take you to jail for it. So it's really important to follow the rules and be a good person so you don't get in trouble with the police.