Gist: Plot against and the whole motion becomes one curve. Height is where the object is; steepness is how fast it is going.
Schematic, redrawn from p.2 — the PDF is the authority on the exact shape.
Why it matters: It turns velocity into something you can see — the slope of the tangent at time . Rising curve, positive velocity; flat, zero; falling, negative. The five sampled times are the ones average velocity is worked through on p.1, and the ordering is what makes those cases come out: .
Detail: Lecture 2, p.2
Related: Instantaneous velocity, Average velocity, Physics