Certain actions will get the player sent to solitary confinement, which is a designated cell that contains only a bed and a security camera.
While in Solitary, the game skips three days, and all your skills will decrease. Contraband items that you've placed in your prison cell will be confiscated, and you will lose all your inventory items,
Reasons for going to Solitary
Guards discovering contraband in your cell
Guards discovering evidence of tunnelling, holes in walls ect
Backup arriving after a lockdown