A custodian is a general term that describes someone responsible for something.  All controlled information and system components need to be controlled.  The custodian(s) are assigned control and responsibility for system components and physical and electronic controlled information.  A custodian is responsible for physical access to the organizational spaces and information resources, physical or electronic.  Control can be assigned to others (i.e., custodian(s)). Control is through a documented chain of custody.  The goal is an established clear chain of control and responsibility so that at any point in time, the device or physical information is accounted for by the custodian.  It is also to ensure spaces or facilities are under the control of a custodian.
Source: 171Comply.com