flyback diode across the relay coil
In layman's terms, a current through inductance cannot change suddenly, so even if supply voltage is cut off, the energy stored in the inductor would still like to keep the same current flowing. It must generate voltage over the inductor to do that, and this can be limited with the diode over the inductor. The current has a path via the diode and voltage is limited to the diode forward voltage until the current has ramped to zero.