y = a constant
a horizontal line that passes through all points with y-value equal to that constant
x = a constant
a vertical line that passes through all points with x-value equal to that constant
i do not understand where you have difficulty.
skeeter told you exactly what you need to know.
lets use an example, i suppose
example 1: say we want to graph y = 1
this is just a horizontal line that cuts the y-axis at 1
example 2: say we want to graph x = 1
this is just a vertical line that cuts the x-axis at 1