"Open justice" means that when important things happen in a court or courtroom, anyone can watch it happen, learn about what's going on, and report on it. It's like a big game, but instead of playing, people are watching and learning.
When something like this happens, everyone wants to know what's going on, and that's why open justice is important. People can find out about what's happening and make sure justice is served.
If we didn't have open justice, only certain people would know about what was happening, and that wouldn't be fair. With open justice, everyone gets a fair chance to learn, watch and understand everything that's happening.