The relay in the image contains a clamping diode to prevent damaging voltage spikes when a coil's magnetic field collapses. This voltage spike can damage its sensitive control circuit. A clamping diode allows current to flow in only one direction. Voltage is allowed to pass to the ground but prevented from suddenly passing through sensitive upstream circuitry.
Diodes are formed from different types of material that allow current to flow in one direction but not the other. Due to the one-way nature of the relay's clamping diode, it must be inserted in only one direction. Diodes can be placed in one of two positions. They may be placed in forward bias to allow current to pass through or reverse bias to prevent its flow. The relay's clamping diode is placed in reverse bias, preventing the sudden flow of current from passing to the control circuit and shunting it to the ground instead.