Martin Luther was a Roman Catholic monk who taught in Wittenburg, Germany in the early 16th century. For years, the guilt of his sin overwhelmed him, and the means the church provided for penance didn't relieve him of his guilt. Reading the library Bible one day (Bibles weren't widely available at the time), he came across Romans 1:16-17 and he understood the Gospel as he never had before. "The just shall live by faith." Reconciliation with God was by grace alone through faith alone.
This sparked Luther to examine a number of practices of the church he thought should be discussed and reformed. He developed a list of 95 points, gave them to his bishop, and nailed them to the church door on October 31, 1517, which was the common way of calling for a community discussion. But the debate wasn't welcomed by the church and, instead, Luther and others who agreed with him ended up separating from the church.
The Reformation was significant for a number of reasons. Luther translated the Bible into German, and because the printing press was coming into use at the same time, individuals were able to have the Bible and read it themselves for the first time. This affected literacy - many communities taught their children to read so they could read the Bible.
The theology taught by the Reformers placed authority in the Bible not the church, so this elevated the individual. This, in turn, was the worldview that was fertile ground for democracy and human rights. And it eventually was the motivation for Pilgrims to look for a place to practice their religion in peace. The Reformation was signficant theologically but also socially.