Summary: Stall Current is the amperage drawn by a motor when its armature is prevented from rotating. The stall current should be less than that of the decoder.
The current that an electrical motor draws when its shaft if prevented from moving. This is typically the highest current a motor will draw. The stall current of a locomotive should be less than the peak current capacity of the decoder that is fitted. If more than the rated current of the decoder, should the mechanism bind or jam, the decoder will be destroyed by the excessive current.
When a motor is operating, the armature is spinning in a magnetic field. This creates eddy currents in the armature. These eddy currents are opposite in polarity and oppose the flow of current in the winding. More powerful magnets mean stronger eddy currents, which limits the speed and current draw of the motor.
If you prevent the motor shaft from turning, no eddy currents are created (the armature must be moving in the magnetic field). The only limit to the current flow is the Series Resistance of the windings in the motor, so the current will increase.
This can be demonstrated using an ammeter and lamp in series with the motor. While spinning the ammeter will indicate current flow and the lamp will glow. Grab the shaft and prevent the armature from spinning, you will see the current has increased by the indication on the ammeter and the brightness of the lamp.