Do-gooder derogation is a term used to describe the negative feelings some people have towards others who try to do good things. Let's say you have a friend who always wants to help people, like giving money to charity, volunteering at a shelter, or picking up litter. Another person might feel annoyed or even angry towards your friend because they think your friend is doing good things just to make themselves look good or feel superior. This negative feeling towards people who do good things is called do-gooder derogation.
It's like when you're in school and you have a classmate who always raises their hand to answer the teacher's questions. You might start to think they are doing it just to show off or make the class feel dumb. But maybe they really like the subject and want to share their knowledge. It's the same with do-gooders - some people might think they are trying to show off or make others look bad, but really they just want to help make the world a better place. It's not fair to judge someone just because they are trying to do something good.