A pun is a play on words. The words in a pun have more than one meaning, they may sound similar but have different meanings or be humorous themselves.
In the sentence above, cell is the word used as pun. Cell has double meaning: it can be a small room in a prison or the smallest unit in an organism.
Here, the pun is intended to stress the significance of the word prison, the fact that the body is made up of cells is compared to the fact that each prison is made up of cells also, therefore, the person has the right to feel that way.
