If the line is parallel to the x-axis then it is a horizontal line with slope 0. These lines have form y=b where b is the y-intercept or y value of all points. y=-3 is the line since (2,-3) is on the line.
You know how a puppy is a kind of dog, but not all dogs are puppies? Well, the same thing is true for lots of other categories of things, including squares and rectangles.
