Now, AC current can have many 'phases' (2-phase, 3 phase, 4-phase etc). The common format used for power supply is 3-phase AC power. ( I can explain this if required). Of these 3 phases, only 1 phase is sufficient for operating most common things. You can see this in most household connections. Some houses will have all 3 phases, but some will have only 1 or 2 phases (out of 3).
There are several types of AC motors, most notably 'synchronous motors' and 'induction motors'. Of these, Induction motors are very suitable for railway purpose, as the speed of rotation of the motor can be changed by changing the frequency of the AC voltage.
Now...
more... all the AC-AC locomotives in India use 3-phase induction motors. But the OHE carries only 1 phase. The electronics on the locomotive convert the 1 phase from the OHE into 3 phases and feed it to the motor. Hence these locomotives are called '3-phase' locomotives.