Christianity and colonialism is a complicated topic, but here's a simple explanation.
Christianity is a religion that began over 2,000 years ago. People who followed this religion believed in one God, Jesus Christ, and followed his teachings.
Colonialism is when one country takes control of land and/or resources from another country. This was a common practice during the 1600s and 1700s.
The two often meet when European countries colonized places around the world. This means that these countries took over the land and the people of those countries. As part of this, the European countries tried to spread Christianity to the people they colonized. They did this by requiring the people to follow Christian beliefs, building churches and for some, even punishing people for not following the religion. This often caused issues between the local cultures and beliefs and the beliefs of the colonizing countries.