FUN stands for Family Unity Night, or simply Family Unity for short, and consists of a group of parents of elementary school aged children. Adults gather for fellowship and conversation and often children are welcome and enjoy their own supervised activities. Gatherings frequently feature a guest speaker, small group discussions, and/or a potluck meal at a member's home.
This is anime made simply because the producers thought it would be fun to do. Tends to be high energy, humourous stuff. It might not have much depth, but a bit of flash and pace can make a nice change now and then.