An Army internment camp, in this context, is a camp where people of Japanese descent living on the West Coast were sent during World War II. In this context, “internment” refers to the process of being kept in confinement without trial and without being suspected of committing a specific crime.
In the early part of World War II, the American government was very worried about the possibility that people of Japanese descent living in America (I cannot call them Japanese-Americans because not all of them were American citizens) would help Japan in the war. The government feared that they would help Japan, for example, to attack or even invade the West Coast.
Therefore, the government rounded up all of the people of Japanese descent on the West Coast and send them to internment camps in the interior of the country, away from the coasts. They were treated better than if they had been in prison, but they still lost their freedom and much of their property.
This is what happens to Yuki’s father in Journey to Topaz. He is arrested before the more general internment and sent to a camp in Montana.