When physicists want detailed
information about the movement of an object, they gather
data about the object each second or millisecond. Sometimes
they gather the displacement (distance and direction from a
starting point) at each moment in time
and other times it is more useful to record the object's
velocity.

The data collected is then plotted
with the independent variable, time, on the x axis and either
the object's distance and direction or its velocity on the
vertical axis. Plotting this data gives the physicist a
visual representation of the object's motion but the slope
of a motion plot provides even more information.

Here's how it
works. You'll probably remember that the
slope of any plot is ΔY/ΔX. Therefore the slope of
a displacement vs time plot at any given period of time
would tell you about the velocity (Δd/Δt) of the object during the interval
you are concerned with.
On the other hand, the slope of a velocity vs time plot during a period time
would reveal the object's acceleration. This is because
acceleration is ΔV/Δt.