Colonialism has to do with one country exploiting another country by making it into a colony. Which is usually good for the mother country and bad for the colony.
Colonialism is a system in which one country creates an empire by taking over other lands and making them into colonies. Colonies don't have power over themselves, and the mother country takes resources and money from the colony. It's not a very fair system, but it's also how the United States got its start: England was really into colonialism and had built a huge empire, until some feisty American patriots decided to rebel.