Final answer:
In a velocity-time graph, a negative value on the y-axis signifies that the object is moving in the opposite direction of the defined positive direction.
Step-by-step explanation:
A negative value on the y-axis in a velocity-time graph indicates that the object is moving in the opposite direction relative to the positive direction defined for the scenario. For example, if forward movement is considered positive, a negative velocity means the object is moving backward. On such a graph, a negative value does not mean that the object is at rest, accelerating, or moving with constant velocity unless accompanied by other information about the slope of the graph (for acceleration) or horizontal linearity (for constant velocity).