Could somebody explain second person to me?

What is the “narrow cell” in line 15 of “elegy written in a country churchyard”

What does water came out from a rock mean in this poem and why is it compared to the radio waltz? time was away and somewhere else. the waiter did not come, the clock forgot them and the radio waltz came out like water from a rock: time was away and somewhere else.

Why did martin luther king jr mention the ancient greek philosophersocrates in his letter