Sometimes electrical connections become corroded and the resistance climbs. This in turn heats up the connection and usually causes an increase in amperage, causing the use to blow or breaker to trip. These connections are dangerous and should be cleaned or replaced.
Things like fuses will have little or no resistance when you read the resistance through them. A single-pole switch will have infinite resistance through the connections in the "off" position and no resistance in the "on" position." Testing continuity with a multi-tester can come in very handy in this instance.
Photo: Timothy Thiele