Question

What variable does the slope represent?

Answer 1

The answer to your question is velocity. : ) Let me know if you need anything else!

Answer 2

The y-axis is the distance and the x-axis is the time.
The displacements are the points at the end of the lines.
The main difference between speed and velocity is direction.
Speed would be only the distance divided by the time, but you wouldn't know the direction.
So the answer is that the slope represents the velocity.

Hope this helps!