Value that is measured by the slope of a position-time graph

The slope is the change in y over change in x.

In a position vs time graph, the position is the y-axis, and the time is the x-axis.

So the slope would be position over time. Position is a distance. So the slope is distance/time, which is velocity.

And also looking at units, distance is in meters (m), and time is in seconds (s). The unit m/s is a unit for velocity.