What is the "food temperature danger zone? "

Exactly that. Frozen food is sterile, no bacteria can grow in it (in normal conditions). Very hot temperatures kill bacteria. In between, bacteria can proliferate and provoke diseases.
41 - 140 degrees F. The danger zone is the temperature at which harmful bacteria can grow and potentially cause foodborne illness. These are inhibited when food is kept cold (below 41) or when food is adequately heated (above 140). Keep cold foods cold and hot foods hot is how the saying goes.
Food. Either not too cold or not too hot. In moderate temp, bacteria can and will grow.