Okay kiddo, so a Popish Plot was a big thing that happened in England a long time ago, and a Ballad is a song or a poem that tells a story. So a Ballad upon the Popish Plot is a story that is told through a song or a poem about the Popish Plot.
Now, let me explain what the Popish Plot is. A long time ago in England, people were really worried about the Catholics, who were people who believed in a different kind of religion than everyone else. Some people thought that the Catholics were trying to take over the government and make everyone follow their religion, which made a lot of people scared.
So, some men came forward and said that there was a plot, which is like a secret plan, that the Catholics were making to kill the King and take over the government. This made everyone even more scared, and many people started blaming Catholics for everything bad that happened.
The Ballad upon the Popish Plot probably talks about all of this - how people were scared, how they blamed the Catholics, and how they were worried about the plot to kill the King. It might also talk about how some people didn't believe that there was actually a plot and how everyone was fighting about what to do.
The Ballad might have been used to help people remember what happened during this scary and confusing time, or it might have been used to try to convince people to take a certain side. But either way, it was an important way for people to express their feelings and tell the story of what had happened.