Danger - LOW voltage!The contacts in a mechanical relay or switch consist of two pieces of metal that touch each other so that electricity can flow between them. However, under certain conditions metal contacts can become coated in insulating sulphides and oxides due contamination from the air. This can prevent the contacts from conducting electricity when they are meant to.
When the contacts are controlling high power loads like light bulbs or motors this is not noticeable, because there is enough voltage and current in the circuit to blast away the thin insulating coating. In low power electronic circuits, however, say a digital limit switch input to an electronic controller or PLC, there may not be enough voltage and current to clean the contacts. The circuit will then fail to detect a contact closure, This has been cause of many mystery system failures. The consequences can be very serious indeed.
The solution is to ensure that the contacts' minimum voltage and current ratings match the voltage and current presented by the controller input. This means you don't select a nice rugged tungsten plated 20A switch but one designed specifically for low power operation, say bifurcated gold plate over silver.
The current through the contact, which keeps it clean, is technically referred to as wetting current (or whetting current if you are old school).