Lines that go uphill to the right have slopes which are positive numbers.
Like these:



Lines that go downhill to the right have slopes which are negative numbers.
Like these:



Lines that are horizontal have slope 0  (zero), like these:



Lines that are vertical have NO slope, like these:



Vertical lines are the only lines with NO slope.
All other lines have slopes.

y = 8 is the equation of a horizontal line with slope = 0 
and y-intercept (0,8) and it has no x-intercept.  Like this:



x = 8 is the equation of a vertical line with NO slope 
and x-intercept (8,0) and it has no y-intercept.  Like this:



Edwin