SkiBird is going to try to help you remember how slope applies to straight lines.

SkiBird

Positive
Slope

Lines that have positive slope, slant "up hill"
(as viewed from left to right).

SkiBird has to work hard to make it up the hill.  He needs to use positive (+) energy to get up the hill.

Negative Slope

Lines that have negative slope, slant "down hill"
(as viewed from left to right).

SkiBird enjoys the ride down the hill.  He needs to occasionally use negative 
(-) energy to try to slow down. 

Zero
Slope

Lines that are horizontal have zero slope.

SkiBird is cross-country skiing on level ground.  He is not working hard to get up a hill, nor is he trying to slow down.  His energy level (and his enjoyment level) is at zero. 

No Slope or
Slope Undefined

Vertical lines have no slope, or undefined slope.

SkiBird cannot ski vertically.  Sheer doom awaits SkiBird at the bottom of a vertical hill.