Dispensation is like when your mom lets you stay up late on a school night, even though you're supposed to go to bed early. In the Catholic Church, dispensation is when a bishop or some other important person lets people do something that they're not usually allowed to do.
For example, there are certain rules about getting married in the Catholic Church. You're supposed to get married in a church, and there are certain things you have to do during the ceremony. But sometimes, there are good reasons why people can't get married in a church or can't do things exactly the way they're supposed to. Maybe they're getting married in a different country and can't find a Catholic church nearby, or maybe one of them is a soldier and has to get married quickly before they're sent overseas. In cases like this, the bishop can give them a dispensation, which means that they're allowed to get married in a different place or do things a little differently than usual.
Dispensations can also be given for other reasons. For example, there are certain days of the year when Catholics are supposed to fast and not eat as much as usual. But some people might have health problems that make it hard for them to fast, or they might have really physically demanding jobs and need more food than usual. In cases like this, the bishop can give them a dispensation, which means that they're allowed to eat more than usual on those days.
So, dispensation is like a special permission slip from someone in charge that allows you to do something that you're not usually allowed to do. In the Catholic Church, it's used for things like getting married in a different place or eating more than usual on a fast day.