Okay kiddo, let me explain what this all means.
The government accountability office is like a group of people who make sure that the government is doing their job correctly. They check to make sure that everything is going smoothly and that the government is spending money right.
Now, the department of defense is a part of the government that helps protect us and our country. They need to have money to buy things like weapons and equipment to do their job.
The government accountability office does investigations of the department of defense to make sure they're spending the money they get correctly. This means they're checking to make sure the department of defense isn't wasting money or doing something wrong.
Basically, it's like the government is getting checked up by someone else to make sure they're doing their job right.