Please explain the difference in meaning between these two sentences. Not able to make out the difference.
He's in prison.
He's in the prison.
The phrase "He is in prison." means that he is confined in a prison somewhere. His actual physical location is not important. This phrase indicates his status as a prisoner.
The phrase "He is in the prison." refers to his physical presence in a specific prison complex. The speaker may be standing inside or in front of the prison. He means that someone is inside the prison right now, possibly on business or as a visitor.
"In prison" holds the additional connotation of being in a prison as a prisoner. So you wouldn't use "in prison" to refer to, say, one of the officers working there, or someone visiting.
(I'll add that "in jail" vs. "in the jail" is the same.)