Imagine you have a bunch of cookies, but you don't remember how many you had to begin with. You know you had 20 cookies yesterday and now you only have 10. You don't know what happened to the other 10 cookies. This is kind of like what "material unaccounted for" means.
But instead of cookies, we're talking about things like fuel or chemicals that are used in big factories or power plants. Sometimes, people who work at these places can't explain where those things went. Maybe there was a spill, or maybe someone used more than they were supposed to. Whatever happened, the amount that's missing is called "material unaccounted for."
Companies and government agencies try really hard to keep track of all the materials they use because it's important for safety and efficiency. So when they can't explain where something went, they investigate to make sure nothing dangerous or illegal happened. Like when you try to figure out where your missing cookies went, your parents might ask your siblings if they took any without asking, or if the dog got into the cookie jar.