This simply means that the equation of the line is a horizontal line at the appropriate y value, in this case 49.
F(X) = 49
Y = 49.
Draw a horizontal line on the y axis at the 49th number, basically, the X coordinates of all other points, would vary, but the y coordinate is the same for all of them. As the graph continues.
Domain is all real number
Range is not, it would be 49 only.