During World War II, some parts of the world were under the control of countries that weren't their own. These areas were dioceses and were overseen by leaders from the Catholic Church.
However, some of these leaders were no longer able to safely carry out their duties due to the war. This meant that the dioceses were at risk of not being properly looked after.
To fix this problem, the Catholic Church decided to reorganize the occupied dioceses. This basically means they moved around the leaders that were still able to carry out their duties to different dioceses so that all areas were being properly looked after.
Think of it like a game of musical chairs, but instead of sitting down when the music stops, the leaders just move to a different seat to make sure everyone is taken care of.