Sometimes it takes a long time for a character to get a joke. Missed in-jokes and Parental Bonus can mean someone gets the joke much later. Or perhaps the character is just slow of mind. At the time the character may not even realize it was a joke. This trope is what happens when, several scenes, in-universe years, or even seasons after the original joke was told, the clueless character finally understands the joke and gives off a great big laugh about it. Irony makes this especially funny if the actual joke had to be repeatedly explained in the process of all this.