It means that the contact has no voltage attached to it. This contact can then be used as an input for a system separate than the one with the dry contact, and can be used with a separate power source.
This is a generalization but I would say most dry contacts are rated for use up to 250V.
For example, lets say the controller on my system, and all of main relay outputs operate on 24VAC. I have a seperate alarm contact that has an isolated common. You could then take my system, and hook up the alarm contact to a 120V light bulb, or to a 5VDC input on a PLC, etc.