Question 697930
The idea behind graphing is that no matter where you are
on a plane surface, all you need is 2 numbers in order
to tell someone where you are.
Suppose you are on a flat plane with nothing in sight to
use as a description of where you are. Then you notice there are 
2 straight lines intersecting at right angles that extend to
infinity. 
Now all you have to do is measure your distance, (a) from one line
( call it the y-axis ), then measure your distance, (b) from the
other line ( call it the x-axis ). Now your position is:
( x[1] , y[1] ) = ( a , b )
where a and b are the measurements you made.
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Now your problem is to really understand what {{{ y = -1 }}}
means. I just told you that you needed 2 measurements to
locate a point on a plane, so this can't be a point. 
What it is is a line, but this line always sees {{{ -1 }}} when
it looks at the y-axis, but it can see anything when it looks at the x-axis.
So this is the line {{{ y = -1 }}} which is parallel to the x-axis
and intersects the y-axis at the point (0,-1). 
So if you pick a point at random on this line, you see a random point on the x-axis,
but you see {{{ -1 }}} on the y-axis
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Hope all this helps. It wasn't easy for me, and the
only way to fully get it is to just keep working
problems or comprehension exercises.