Velocity is the steepness (or slope) of a distance graph. Since this graph is not linear, slope can only be approximated.
Think about ramp lab (2-112). Slope at a specific point was approximated by finding the slope of a line that connects two points close to the target.
We can approximate the slope at t = 1 by choosing two points close to t = 1 and finding the slope of the line that connects them.
This is a distance graph. y-values represent distance from the classroom.
The classroom is located at y = 0.