The text you cite has been attributed to Pastor Martin Niemoller (1892–1984). Neimoller was a German minister who at first supported Hitler's rise to power. Later, when Hitler declared that the state had greater authority than the churches, Neimoller rebelled and became the leader of German clergymen who opposed Hitler. In 1937, he was arrested and sent to a concentration camp.
Neimoller is expressing the idea that it is not acceptable to remain silent when other people are being persecuted. His reasoning is very simple: eventually, you will be the one who is persecuted, and then who will speak up for you?
Niemoller mentions three groups who were persecuted by the Nazis before they persecuted him:
c) trade unionists.
Part of Nazi ideology was opposition to Communism. Under Communism, the government owns the "means of production," meaning all important industries and businesses. The Nazis, by contrast, believed in the importance of private ownership of the means of production. Under Nazi rule, many proponents of Communism were persecuted, imprisoned, or killed.
The Nazi persecution of Jews is well known and need not be explained again here.
"Trade unionists" are workers who have organized themselves into unions to protect their rights and to negotiate for better salaries and working conditions. Although not all trade unionists are communists, the union movement is often seen as being tinged with communism. Thus, the Nazi persecution of trade unionists goes together with their persecution of Communists.