To understand it you must understand the graph.

The only lines with undefined slope are vertical lines. 

First draw the x-intercept, which is the point on
the x axis marked 2.  It has the coordinates (2,0):

 

Draw a vertical line through that point:



Now look at some more points on that vertical line:



Look at all those points, 

(2,4), (2,3),  (2,1.5),  (2,0), (2,-2), (2,-3)

What do they have in common?

Well, they have different y-coordinates, but THEY ALL HAVE
THE SAME x-COORDINATE! And that x-COORDINATE is 2.

So to describe that line we could say:

"The line on which every point has x = 2"

That's why we say the equation of the line is

x = 2.  That's because the x-coordinate is always equal to 2.  

Edwin