Well, kiddo, colonialism is when a country or a group of people go to another country and take it over or control it. They usually do this by using force, and they might make the people who already live there do what they say.
Genocide is when one group of people tries to kill all the members of another group of people. Genocides often happen when one group thinks they are better or more important than the other group, and they want to get rid of them.
Unfortunately, colonialism and genocide often go hand in hand. When one group of people takes over another group's land or country, they might think that the people who already lived there are less important or valuable. They might try to get rid of them, or make their lives really hard.
Sometimes, this even leads to genocide. The colonizers might try to kill all the people who lived there before, or force them to leave their homes and never come back.
This is really sad and unfair, and thankfully not very common nowadays. But it's important to remember that it's not okay to treat people differently just because they come from a different place or have a different background than you. Everyone is important and should be treated with kindness and respect.