World War II was a total, world war because it involved countries around the entire world and involved them in such a way that they had to use their entire economies and populations to participate in the war.
Unlike World War I, WWII was a truly world war. WWI involved a few non-European peoples, but largely because those peoples were connected to European imperial powers. Japan was officially in the war but did very little. The vast majority of the fighting occurred on the European continent. WWII was not like this. Japan was, of course, completely involved in this war. Fighting occurred in China and on many Pacific islands. It took place in Northern Africa as well. It was a much more widespread war than WWI.
WWII also involved more of the belligerents’ populations. In the US, for example, there was rationing because many commodities were needed for the war effort. Women went to work in roles that had previously been closed to them. The same sorts of things happened in Japan, for instance. In Japan and Germany and England, civilians far from battlefields were killed in large numbers by bombing raids. These sorts of things were much less common in WWI.
For these reasons, WWII was global and total in a way WWI had not been.