So, a long time ago, there were two big countries called Germany and Soviet Union. They didn't get along very well and they fought a big war. When the war was over, the Soviet Union was pretty unhappy with Germany for starting it. So they made a rule that all of the Germans who lived in parts of Germany that the Soviet Union took over had to work for the Soviet Union. This was called forced labor.
Forced labor means that someone has to work for someone else, even if they don't want to. A lot of Germans didn't want to work for the Soviet Union because they would rather be with their families and friends in their own homes. But the Soviet Union made them anyway.
The Germans had to work on things like building roads, working in factories, and farming. The Soviet Union wanted them to work hard and they didn't always treat them very nicely. The Germans didn't have a choice in the matter, they had to work or else they would get in trouble with the government.
Forced labor is not allowed now because it's not fair to make someone work for someone else without their permission. It's always important to treat everyone with respect and allow them to make their own choices.