Plot some points.  Make a chart like this

x | y
-----
  |
  |
  |

Now since the equation is y = -1, the only
choice we can give for y is -1, so we put
-1's for all the y's

x | y
-----
  |-1
  |-1
  |-1

The equation y = -1 contains no x's, so we
can pick any three numbers at random for
the x's.  Say we pick -3, 0, and 4

x | y
-----
-3|-1
 0|-1
 4|-1

Plot those three points



Draw a straight line through them:



So you see it is a horizontal line through -1 on the
y-axis.  

Edwin