An incentive trust is like a special treasure box, but instead of giving the treasure all at once, it gives a little bit at a time. This is because the person who made the treasure box (the person who has all the money) wants to make sure that the person who opened the box (who will get the money) will work hard and accomplish certain things that the creator thinks are important.
For example, let's say your grandma made an incentive trust for you. Every time you finish a school year with good grades, your grandma will put a bit of money into the "treasure box" for you to use later. If you get into college, she'll add a lot more. If you graduate with a good degree, she'll add even more.
This can motivate you to work hard in school and make good choices because you know that if you do, you'll get a reward. It's like a game where you have to complete certain tasks to unlock new levels and rewards.
Overall, an incentive trust is a way for someone to encourage good behavior or accomplishments by giving a reward in the future.