For those who may be less than conversant with things electronic, here's what they are talking about.
When voltage is applied to the coil of a relay, current flows through the windings that cause magnetic lines of flux to radiate from the coil wires which causes a magnetic field (the windings on the coil are designed in such a way to cause these lines of flux to be additive). The core of the windings then becomes a magnet as long as there's current flow through it. When voltage is removed, these lines of flux collapse back and as they do so, they pass through other coil windings to causes an "inductive kick" or voltage spike to be generated that can be many times the voltage of the original voltage source (often thousands of volts, but with relatively low current capacity). This inductive kick can travel back down wiring to cause arcing at the switch contacts and to sometimes do damage to sensitive components, or to shock you if you happen to be touching the contacts.
Placing a simple rectifier diode across the coil contacts (in a reverse direction to the normal relay coil current flow - i.e. with the banded or negative diode lead to the positive relay coil terminal etc.) will suppress (short out) the inductive kick.
