The 1N4004 (sometimes referred to as an IN4004) is a high current capability and low forward voltage drop diode usually deployed across the terminals of a coil to prevent flyback voltages from destroying the coil driver circuitry.
When you remove power from a coil, be it a flipper coil, relay, or any type of heavy duty magnetic coil, the collapsing magnetic field can induce a significant voltage in the coil windings. Unless properly handled, this flyback voltage as it is called, will overpower the output stages of the driver circuit, possibly smoking your TIP102 or MOSFET coil driver.
To avoid this, use a flyback diode such as the 1N4004 to shunt the high voltage direct to ground and away from the drivers.
The 1N4004 is capable of handling up to 30 amps and up to 400 volts (peak reverse voltage). Ideal for connecting across your pinball coils.
Sold as a pack of ten.