Imagine you have a toy box and it is filled with all of your favorite toys. Now, pretend that you want to find your favorite toy car, but it's not in the toy box. You start looking around your room and under your bed, and finally, you find it!
That feeling you get once you find your toy car is similar to what happens when a case is closed. People go through a lot of information and do a lot of talking to try to figure out what happened, and when they finally come to a decision, the case is closed. It's like they found what they were looking for, just like you found your toy car.
Once a case is closed, it means everyone has agreed on what happened and what needs to be done. This could mean that someone is going to get in trouble for doing something wrong, or it could mean that something has been fixed and is now safe again. Basically, it's the end of a story and everyone knows what happened.