Heresy is a belief that isn’t part of the official doctrine of a religion. In the Middle Ages, heresy was a crime and taken very seriously. The church was threatened by heresy, and cracked down on it by punishing the heretics. Punishments ranged from penance to death. Sometimes, one could be sent to go on a pilgrimage as penance. Excommunication was the second worst punishment from death. If you were excommunicated, everyone would have to ostracize you or be ostracized along with you. In the Middle Ages, that means that no one would help you. No one would give you shelter if you needed it, or food, or any kind of attention. If you fell on hard times, you would probably starve because no one helped you. Proving one’s innocence was also tricky, as it was decided that God would save someone if they were innocent, and you had to do stuff like sticking your hand in boiling water! There were two types of heresy, formal, which was deliberate and material, which was out of ignorance. Christianity was central in Medieval life, and heretics undermined it according to Medieval people. The church was very powerful and heretics were undermining that power too.
(Is it just me, or does that guy have a sadistic smirk on his face? ;)