Kampuchea Krom is an area located in the southern part of Vietnam, which used to be a part of Cambodia. It's like if you and your friend had a toy, but one day your friend takes it and says it's theirs. That's what happened with Kampuchea Krom.
Cambodia used to own the area and people there spoke the Cambodian language, had Cambodian customs and traditions, and followed Buddhism. But a long time ago, Vietnam took control of Kampuchea Krom and forced the people there to follow Vietnamese customs and learn the Vietnamese language. It's like if someone came and made you start speaking a language you didn't know or follow rules you didn't like.
People who want Kampuchea Krom to be part of Cambodia again are upset because they think that Vietnam is not treating the people who live there fairly. They want the people of Kampuchea Krom to be able to follow their own customs and traditions and speak their own language without being forced to do anything else.
It's a complicated issue, but the main idea is that Kampuchea Krom used to belong to Cambodia, but now it's part of Vietnam and some people are not happy about it.