This quote describes the way in which the Nazis operated during the Third Reich. The quote comes from the German pastor Martin Niemöller, and talks about the cowardice and individualism that the pastor observed during these years.
The "they" that the poem refers to is the Nazis, while those who did not "speak out" were the people that were not persecuted. The quote tells us that many people did not feel compelled to help others because they were not in the same social group. However, this also meant that no one helped them when they ended up being in the targeted group.
