A more thorough explanation:

Definition: Jail liberties refer to the bounds within which a jail or prison lies and throughout which certain prisoners are allowed to move freely, usually after giving bond for the liberties. These bounds are considered an extension of the prison walls. Historically, jail liberties were given in England to those imprisoned for debt. The prisoners were allowed to move freely within the city in which the prison was located.

For example, in Boston, the whole city was held as the "gaol liberties" of its county gaol. This meant that prisoners who were granted jail liberties could move freely within the city limits while still being considered within the walls of the prison.

This concept of jail liberties was created to mitigate the hardships of imprisonment and provide some freedom to prisoners who were not considered dangerous or a flight risk.