Leakage flux refers to the magnetic flux produced by the armature current that does not cross the air gap and follows a different path. It does not contribute to the useful flux that interacts with the field in the air gap. This type of flux can lead to additional losses and lower the machine's efficiency.
On the other hand, the main flux is the useful flux that crosses the air gap, interacting with the rotor or stator to generate torque or electrical power.