I have to modify an existing PLC Ladder Logic program but I have no experience in programming the PLC and at this point it is little hard for me to comment on the code as I don't know what it exactly does. Can someone please suggest any reference material from which I can educate myself on programming PLC's ?

Your best be t is the manufacturer's literature.

I can tell you this, it's reminiscent on relay ladder logic, BUT in this case the RUNGS of the ladder are executed very quickly.

Simple rung: Switch turns on light.

More complex program:

Rung 1: Push button switch starts a one-shot timer which as 1 contact named NO#1
Rung2: NO#! Is a contact that turns on light.

So, light turns on after applicable delay.

I have 20 years experience in PLC programming. Your question is too general for specific answers.

The answer needs to start with the manufacture of PLC in question.

If you are still looking for an answer let me know.

Carl