What is the difference between the two sentences?
1. He was arrested on charge of theft.
2. He had been arrested on charge of theft.

The police arrested him on a charge of theft.

The police had arrested him on a charge of theft.

Passive Voice:
He was arrested (by police) on a charge of theft.

He had been arrested (by police) on a charge of theft.

One is Past Tense, Passive Voice, and the other is Past Perfect, Passive Voice.

My question is what is the difference between simple past and past perfect? Is there any difference of what the two sentences mean? If yes then what is the difference? If not then are simple past and past perfect same ? Actually I want to know when we use simple past and when past perfect?

I don't hand feed. There are plenty of grammar websites going into this in detail - have you looked them up?
If so, just what, specifically, don't you understand about the difference?

Put some of your own effort into learning.