Okay kiddo, so you know how when someone does something wrong, they might have to go to jail? Well, when they are in jail, they might behave really well and show that they have learned their lesson. When that happens, the people in charge of the jail might decide that they can leave early, but only if they promise to keep being good and not do anything naughty again.
This promise is called a "ticket of leave." It's like a special permission slip that says the person can leave jail before their sentence is finished, but they have to follow certain rules and behave themselves.
If the person follows the rules and doesn't get into trouble again, they might eventually be able to leave jail for good and go back to their regular life. But if they break the rules, they might have to go back to jail for even longer.
So, a ticket of leave is like a chance for someone to leave jail early if they have proved that they can be good and stay out of trouble.