Current flows from Positive (+) to Negative (-) to complete a circuit. This current is what operates the light, motor, etc. as it travels through the component searching for a ground - if it doesn't find ground, the component will not operate.

A relay has two circuits going through it, a low amp, switched circuit (pins 85 and 86) and a high amp power circuit (pins 30 & 87).
The low amp circuit will either connect or disconnect the high amp circuit in the relay. The current will start with Positive signal and will go through the relay searching for a Ground. Between the relay and the Ground it is looking for, is a "switch" albeit a toggle, temp or pressure type switch that will stop the current flow - once this "switch" is activated, it will close and allow the current to connect to Ground, completing its circuit.