A vertical line is a line that is straight up and down. It doesn't tilt. Think of a flagpole as one of many examples.
I suggest plotting the point (-0.4, 0.3) on the coordinate grid. Draw a vertical line through this point. You'll see that there are other points on this line such as...
(-0.4, 0) and (-0.4, 2) and (-0.4, 5)
and so on...
There are infinitely many points on this line. All of them have the same x coordinate of -0.4 since that's what the original point has. Y can be anything you want.
So the equation of this line is simply x = -0.4 to indicate "force x to be -0.4 and y can be whatever you want"