Unitarianism is a kind of religious belief that says people should only believe in one God instead of three - like in the Christian belief of the Holy Trinity. This means that they don't believe that Jesus, the Holy Spirit, and God are three different parts of one supreme being.
Unitarians believe that the Bible is an important guide to living a good life, but that everyone should have the freedom to interpret it in their own way. They also focus on the goodness of human beings and the importance of doing good deeds and helping others.
Overall, Unitarianism is about being kind and respectful to everyone, regardless of their beliefs or religion, and treating people how you would like to be treated yourself.