In the 1500s there was a great revival in the German countries that centered around the concept of believing and obeying the Word of God as it describes the Christian faith and life regardless of what traditions and current authorities practiced. The idea was that God inspired the Bible, and that alone should be the our authority for matters of religion and faith. This is often referred to as the "protestant reformation". They "protested" bad and false practices in the Catholic Church and "reformed" practices to correct the errors.
The most famous "reformer", Martin Luther, along with other fairly well known men such as Zwingli did not entirely follow scripture alone. For example, Zwingli in his early ministry taught that there was no scriptural evidence or instructions to baptize infants. (The practice of the Catholic Church.) But political pressure, which was life threatening in that day, seemed to modify and "tone down" the mainline reformers' teaching. But there were many who were totally sold out to the concept of obeying what was clearly taught in Scripture. These men and women often paid for their obedience with their lives.
The term Anabaptist was used to describe and define certain Christians during the Reformation. These Christians rejected infant baptism, choosing instead believer's baptism. Since many of them had been baptized in their infancy, they chose to be rebaptized as believing adults. So their enemies called them Anabaptists -- "re-baptizers."
Baptism was not the only issue they were trying to reform in the Roman Catholic Church. At that time there were many sinful atrocities committed by those in Church authority. Of course such men were not true followers of Christ. Sometimes in misled zeal and other times with evil malice, greed and deceit; endowed with government authority, they enforced their doctrines with the sword.
These peace loving (pacifist) Anabaptists were persecuted, tortured and put to death not only by the Catholic Church, but by the protestant reformers as well! The injustice, cruelty and darkened hearts are almost unbelievable. It just makes me glad that God is the judge and He will sort all that out at the judgment.