When a person knows a lot about something, we call them an expert. Sometimes, experts get asked to help out in a situation where people need to know about that thing they know a lot about. This could be a court case, where someone needs to explain something complicated to a judge and jury. That's when they become an expert witness.
Sometimes, things happen that are out of our control and people die. When an expert witness dies, it means they are no longer alive. This can be a problem because their knowledge and expertise can't be used anymore to help people understand something. It's like having a library book that you really need to read, but then the book gets lost forever. You can't get the information from that book anymore because it's gone.
While it's sad when anyone dies, it's especially hard when an expert witness dies because their knowledge and skills can't be replaced easily. It means that the people in the court case or situation may have to find someone else to help them, but that person might not know as much as the expert witness who passed away. So, the death of an expert witness is something we try to prevent from happening, but when it does happen, it can make things more difficult for everyone involved.