The integer 0 represents ground level
Any positive number is some distance above ground. Example: the number 5 means you are 5 units above ground level.
Any negative number is where you're underground as the picture shows.
The value 0 is right in the middle. It divides or forms a wall between the positive and negative numbers. So naturally 0 is a good place to have the ground. You can think of it as floors in an elevator if it helps.
(sorry its blurry)