This jail is the place where criminals, who are arrested by the Dimmsdale Police, are brought in and incarcerated. The jail and police department seem to be the same building.
In its first appearance, the jail is a simple square-shaped concrete building with guard towers and barbed wire across the top of the walls, and only a toilet in each cell.
After Timmy Turner wished himself into an adult in the episode, The Big Problem!, he was eventually caught picking on Francis in an act of revenge, and then later runs into Vicky, who does not recognize him and kicks him to the curb when he approaches her, allowing the police to catch him and bring him to the prison. There, Timmy begins to cry when he realizes that Cosmo and Wanda are going to be taken away from him for being too old, but because he was acting like a kid in that moment (as he really wanted his parents here which is something that only children would want), his godparents are allowed to grant him one last wish, and he wishes himself back into a kid, allowing him to keep his godparents. The reassigning fairy is then forced to become the mean kid's fairy godparent. Timmy is then found by one of the police officers in the jail cell and is immediately released (as the officer claims jail is for creepy adults, not for kids). They then pass by some criminals who plead out that they want their mommies and daddies here too. Vicky was at the front desk, looking for Timmy, and when Timmy arrives in front of her, she momentarily rejoices when Timmy is brought to her as this means she is safe from being fired by Timmy's parents.