This 40-second video explains the basics just as well as anything else. It is the study of various themes throughout the bible (there are more than 25 themes, that is just for their study guide). There are traditional themes of discussion where church leaders might call it a doctrine or a principle of God that followers should practice. There were times in history if you did not follow those practices you could be cast out of the church.
Martin Luther is famous for nailing to a Catholic Church door for what amounts to his study of the word. Which was in disagreement with the teaching of his day. Which flamed Protestantism (The rejection of the Pope's authority) another church group or branch of Christianity. That is also generally how other Christian church groups start. Somebody is in disagreement with somebody else.
Theology of Martin Luther - https://en.wikipedia.org/wiki/Theology_of_Martin_Luther
The theology of Martin Luther was instrumental in influencing the Protestant Reformation, specifically topics dealing with justification by faith, the relationship between the Law and Gospel