A site master file is like a big book that a grown-up company keeps to record everything they do at a certain location. Think of it like a scrapbook, but instead of pictures, it has really important information!
This big book tells us everything grown-up companies do at their location, like how they make their things, how they test them, how they store them, and how they clean their place. It also tells us things like who works there, what they are trained to do, and how they keep their area safe.
Imagine you are playing pretend and you have a special place where you keep all your toys. Your grown-ups would want to know how you take care of your toys by themselves, how you clean up after playing, and who you play with in your special place. That's kind of like what grown-up companies do with their site master files, so that they can keep their work safe and do a good job.