The Great Stirrup Controversy was about a disagreement between two groups of people about a tool called a stirrup. A stirrup is a loop of metal or leather that fits around a rider's foot and attaches to the horse's saddle. It helps the rider to stay balanced on the horse and to have more control when riding.
Some people believed that stirrups were a wonderful invention and made it easier to fight on horseback. Other people thought that it was wrong to use stirrups, because they changed the way people fought and they gave some people an advantage.
People argued a lot about this, which is why it's called the Great Stirrup Controversy. In the end, it was decided that stirrups could be used, but it was also important to remember that not everyone had them, so it was important to be fair.