Okay kiddo, so Mary Surratt was a lady who lived a long time ago during a time when there was a big war in America called the Civil War.
Now, during this time, there were some people who wanted the states in America to stay united, and there were others who wanted to split up and have their own states.
Mary Surratt was someone who was friends with a lot of people who wanted to split up and have their own states. Some of these people even plotted to kill the president of the united states, Abraham Lincoln.
Now, it's not nice to plot to hurt someone, even if you disagree with them. Mary Surratt was put on trial because people thought she helped these people plan the attack on President Lincoln.
But, some people thought she was innocent and that the government was just trying to punish someone because they didn't like what she believed in.
After a long trial, Mary Surratt was found guilty and was punished for her part in the plot to attack President Lincoln.