Albert Barnes Commentary Jeremiah 37:21

"Then Zedekiah the king commanded, and they committed Jeremiah into the court of the guard; and they gave him daily a loaf of bread out of the bakers` street, until all the bread in the city was spent. Thus Jeremiah remained in the court of the guard." — Jeremiah 37:21 (ASV)

The prison - the watch (marginal reference).

Piece - literally, a circle, that is, a round cake.

The bakers’ street - It is usual in Eastern towns for each trade to have a special place set apart for it .