Treating people like they're too inferior or uncivilized to be considered real people is dehumanization. Dehumanization can be avoided simply by acknowledging everyone's humanity.
One example of dehumanization might be an employer treating all their workers like robots rather than individuals — for example, by refusing to provide safe working conditions or to allow regular breaks. Thinking of people from unfamiliar cultures as inferior is another kind of dehumanization, as is depriving people of basic human rights, like life and freedom.