Electrical Generators rely on current through the field windings to provide the magnetic field required to generate electricity.
The field voltage is provided by the voltage regulator which in turn is supplied by the generator output.
To get the whole process started, many (older) aircraft generator systems rely on residual magnetism left in the field winding core. If there is no residiual magnetism, spinning the shaft on a generator won't produce the initial output voltage to supply the regulator with the supply it needs to create field current.
Occasionally the residual magnetism is depleted. Usually because of some fault that caused a transient in the field. To get the whole system working again, we need to restore the residual magnetism by applying voltage to the field windings momentarily while the system is NOT running.
You have to make sure that the regulator is not connected too! This process is called "flashing the field".
you are essentially putting power to the field in case of loss of residual magnetism.
This can be done by a small wire from a voltage source, just a momentary contact is all.
Some newer gen's have a warning on the side "DO NOT HI-POT",(do not High potential) that means don't use main battery power to flash it. Flashing of these types of gen's can be done with a simple 9v battery.