What is the exposition of milkweed?
The exposition is the part of the plot of a story (or in this more specific case, a young adult novel) that talks about the setting and the characters. In the exposition of Jerry Spinelli's Milkweed, we learn all about the boy named "Stopthief" and his early journeys in Warsaw, Poland.
The exposition begins as the book opens with either a memory or a dream from the boy now only called "Stopthief." The narrator has stolen some bread and is being hailed as a theif as he runs. As the narrator collapses from exhaustion, he is pulled to safety by Uri who tells him to be more wary of "Jackboots" instead of women guarding their bread. As the town is attacked, Uri leads him to his hideout and introduces the narrator to the other boys under his tutelage. Here, Uri and the other boys learn that the narrator has no idea if he is Jewish or not. Further, the narrator doesn't even know his own name.
In my opinion, the exposition ends at Misha's "birth" (or the giving of his identity by Uri):
And so, thanks to Uri, in a cellar beneath a barbershop somewhere in Warsaw, Poland, in the autumn of the year nineteen thirty-nine, I was born, you might say.
The barbershop cellar is where the boys are hanging out in Warsaw, Poland. It is the fall of very specific year 1939. In this one sentence we learn the exact setting of place (Warsaw Poland, in this scene the barbershop basement) and the exact setting of time (autumn of 1939).