Speed-Time Graphs look much like Distance-Time graph. Time is plotted on the X-axis. Speed or velocity is plotted on the Y-axis A straight horizontal line on a speed-time graph means the speed is constant. This means the speed is not changing over time.
A positive slope represents an increasing speed. The moving object is accelerating.
A negative slope represents a decreasing speed The moving object is decelerating.
The slope of the S-t graph is speed.