Okay kiddo, so you know how we have rules and things we have to do when we play games or do our chores? Well, grown-ups also have rules they have to follow when they work together to make decisions for their city or country. In Hong Kong, there are big people who were elected by the other big people to be in charge of making those decisions. These big people work in a place called the Legislative Council (LegCo for short).
Now, when these big people get elected, they have to promise to follow the rules and do their job well. They take an oath, which is like a special promise that they say out loud in front of everyone. But some of the big people did some things that made other people think they weren't taking their job seriously or following the rules properly.
One of the big people, named Sixtus Leung, used some bad words and insulted China during his oath-taking ceremony. Another big person, named Yau Wai-ching, also did some things that made her oath not valid. This made some of the other big people and leaders very upset and angry because they thought it was disrespectful to the people they were supposed to work with and the country they were supposed to help.
So, the leaders and the courts tried to decide what to do. They said that the oaths for Sixtus Leung and Yau Wai-ching were not valid and they couldn't be in LegCo anymore. This made some people happy, but others were angry because they thought it was taking away their right to choose who represents them.
That's why it's controversy - people have different opinions on whether what the two big people did was disrespectful or if it was fair for their oaths to be invalid.