I’m going to show you what negative voltage is by putting John into a hole. As you’ll quickly learn, it’s nothing weird or mystical.
Meet John. He is 1.8 meters (6 feet) tall.
What does it mean that John is 1.8 m tall? Could you find his height by only looking at his head?
No. You have to compare the top of his head to the ground he is standing on to be able to find his height. That John is 1.8 m tall really means that the top of his head is 1.8 m higher than the ground that he stands on.
It’s the same thing with voltage. You can’t say anything about the voltage without comparing it to another point.