It means that the Holocaust refers to the time period when Hitler was in charge, and was killing and torturing Jews. It lasted from the time Hitler became in charge, until the time that everyone stopped fighting and the war ended. :)
The Holocaust was not widely taught in schools till after 1980 and it was certainly not taught in the immediate postwar period. From the end of World War 2 till the late 1960s the Holocaust was something of a non-subject apart from media reports on Holocaust trials.