I find these kinds of questions get a pretty good answer on Wikipedia:
Humanism is a philosophical and ethical stance that emphasizes the value and agency of human beings, individually and collectively, and affirms their ability to improve their lives through the use of reason and ingenuity as opposed to submitting blindly to tradition and authority or sinking into cruelty and brutality. The term was coined in 1808 by the early nineteenth century German educational reformer and theologian Friedrich Niethammer and gradually adopted into English. Niethammer had wished to introduce into German education the humane values of ancient Greece and Rome. Niethhammer was a Lutheran theologian. Since the twentieth century, however, Anglophone humanist movements have usually been aligned with secularism, and today humanism typically refers to a non-theistic life stance centred on human agency and looking to science rather than revelation from a supernatural source to understand the world.
Humanism, like many other -isms, is an ideology (or perhaps, set of ideologies) that has shifted and morphed over the years, variously including and excluding ideas (such as secularism or non-theism) depending on which proponent of the ideology is speaking.
In addition, proponents and opponents of the ideology tend to emphasize different aspects. For example, a proponent might emphasize "the value and agency of human beings" while either a proponent or an opponent might emphasize that it is "non-theistic" if that would be attractive to the audience. But both are correct in that most forms of the ideology purport to provide a way forward emphasizing the former, while assuming (or in some cases, arguing) the latter.