Why is it necessary to connect PLC output to a load via relay ? Is it mandatory plz help me out ?

It is not “mandatory” to connect a relay to a PLC output.
But you need to connect something that can drive the load you intend to turn on and off with the PLC, because the PLC output is too “weak” (current) and often also a different voltage than what is required by the load. Those “somethings” could also be a MOSFET, a Darlington transistor, a Triac, and so on….
To answer the meanings of the relay board is not easy without actually seeing it. Those “letters” are not standard.
But I assume that the coil is polarized (there’s probably a suppressor diode built-in), so you need to watch the polarity + / – ( “1″ probably just means “relay Nbr.1)
The letter C, O, P could mean that the relay has a SPDT contact, and usually they wold be called
C = C = common
O = NO = open if not energized
P = NC = closed if not energized
Use an Ohm meter and measure between C and NO, and C and NC to verify.