Modern Humanism is a belief that people should lead their life by showing respect to other people simply because they are human. That belief may be based in the religious belief of a supreme creator watching over us, or may be based in the philosophical reasons of simply doing good unto others.
The philosophy behind Humanism does not allow for discrimination of other people. It teaches that all humans should assist and provide for the needs of other humans simply because we are all human. Religious beliefs, citizenship, cultural differences, etc., should never interfere with treating someone else with respect and kindness and assisting them when needed.
Modern religious Humanism associates these beliefs and practices with a supreme creator who has determined good and evil. Modern secular Humanism associates these beliefs and practices with the natural ability of humans to be able to distinguish right from wrong, good from evil, and with basic philosophical human needs to be a good person and to do good works.
The term "Humanism" is not very old but the ideas and beliefs, obviously, go back very far.
I hope this short explanation helps. Please see the link provided for more information if needed.