Potential free contacts (also known as "dry contacts") are simply contacts which are physically operated with the main device, but not electrically connected to it. For example a motor contactor often has auxiliary contacts that are operated by the main coil and open and close at the similar time as the main contacts but are not used for control of the motor starter. If they are connected to an outside circuit to shows the status of the starter without being powered by the motor supply they would be considered potential free contacts.
These type also called Signal-Contacts.

In case of relays and switches if the common/pole is connected internally to a given potential like a 110V phase etc., the contact is not POTENTIAL FREE but if both the common and the normal open (and at time also the normally closed) contacts are presented to the user for connecting in his system as he pleases,the contact is said to be "potential-free"