Humanitarianism is a way of behaving out of kindness to other people. Someone who is a humanitarian is a person who helps to improve the lives of other people without thinking about their race, belief or nationality.
Humanitarianism is not the same thing as Humanism which is about not believing in a god.
A good example of Humanitarianism is the well-known story from the New Testament about the Good Samaritan who goes to help the poor man who has been robbed.
Humanitarianism also means basic morality indispensable for human being. There are international laws called "international humanitarian laws" considered to created out of humanitarianism of this mean. Among them, Geneva Conventions on the matter made at the time of wars are well known.